Glossary
Reference
Glossary
A flat reference of acronyms used across the site. Grouped by topic, not alphabetical. Linked where this site has a deeper page; bare where not.
Core distributed systems
Acronym
Expanded
One-liner
CAP Consistency, Availability, Partition tolerance
Pick two of three under a network partition. See CAP / PACELC .
PACELC Partition: Availability/Consistency, Else: Latency/Consistency
Refinement of CAP that also covers the no-partition case.
ACID Atomicity, Consistency, Isolation, Durability
The four transaction guarantees. See ACID Properties .
BASE Basically Available, Soft state, Eventually consistent
The NoSQL counterpoint to ACID.
2PC Two-Phase Commit
Coordinator-based atomic commit across nodes; vulnerable to coordinator failure.
3PC Three-Phase Commit
2PC plus a pre-commit phase to reduce blocking; rarely used in practice.
MVCC Multi-Version Concurrency Control
Each write creates a new row version; readers see a consistent snapshot. See MVCC .
WAL Write-Ahead Log
Durably log the intent before applying the change; the recovery backbone of every relational DB. See WAL .
LSM Log-Structured Merge tree
Write-optimised storage engine (RocksDB, Cassandra, ScyllaDB).
CRDT Conflict-free Replicated Data Type
Data structures whose merges always converge without coordination. See CRDTs .
CDC Change Data Capture
Stream of row-level changes out of a database. See CDC .
OLTP Online Transaction Processing
Low-latency reads/writes on small rows; classic RDBMS workload.
OLAP Online Analytical Processing
High-throughput scans over large columnar data; warehouses + lakes.
DDD Domain-Driven Design
Modeling complex business logic around aggregates and bounded contexts. See DDD Tactical .
FLP Fischer-Lynch-Paterson
Impossibility result: no asynchronous deterministic consensus with one faulty process.
BFT Byzantine Fault Tolerance
Tolerate arbitrary (including malicious) node behavior.
Consensus & coordination
Acronym
Expanded
One-liner
Raft (algorithm name, not acronym)
Understandable consensus algorithm; leader-based; used by etcd, Consul. See Consensus: Raft & Paxos .
Paxos (algorithm name)
The original consensus algorithm; notoriously subtle.
HLC Hybrid Logical Clock
Physical + logical clock; orders events across nodes.
NTP Network Time Protocol
Wall-clock synchronization; ~10ms accuracy over the open internet.
PTP Precision Time Protocol
Sub-microsecond clock sync within a datacenter.
NWR N, W, R
Replica count, write quorum, read quorum; W+R > N gives strong reads.
Networking
Acronym
Expanded
One-liner
DNS Domain Name System
Hostname → IP resolution. See DNS Resolution .
CDN Content Delivery Network
Edge cache for static assets. See CDN .
TCP Transmission Control Protocol
Reliable, ordered, byte-stream. See TCP Deep Dive .
UDP User Datagram Protocol
Connectionless, no delivery guarantees; lower latency than TCP.
TLS Transport Layer Security
Encrypted transport; supersedes SSL. See TLS & mTLS .
mTLS Mutual TLS
Both ends present certificates; common for service-to-service auth.
HTTP/2 HTTP version 2
Multiplexed streams over a single TCP connection.
HTTP/3 HTTP version 3
HTTP over QUIC; no head-of-line blocking across streams. See HTTP/3 + QUIC .
QUIC Quick UDP Internet Connections
Connection-oriented protocol over UDP, basis for HTTP/3.
gRPC Google RPC
HTTP/2 + Protocol Buffers; bidirectional streaming. See gRPC Mechanics .
ICE / STUN / TURN Interactive Connectivity Establishment / Session Traversal Utilities / Traversal Using Relays
NAT traversal stack for WebRTC. See WebRTC .
CIDR Classless Inter-Domain Routing
Notation for IP ranges like 10.0.0.0/8. See CIDR Subnetting NAT .
NAT Network Address Translation
Maps internal IPs to a shared external IP.
BGP Border Gateway Protocol
Inter-AS routing on the public internet.
OSI Open Systems Interconnection
The 7-layer reference networking model. See OSI vs TCP/IP .
RTT Round-Trip Time
Time for a packet to go and an ack to return.
BDP Bandwidth-Delay Product
The amount of data in flight on a fat pipe; sets TCP window targets.
SSE Server-Sent Events
One-way streaming from server to browser over HTTP.
WS WebSocket
Bidirectional persistent socket over HTTP upgrade. See WebSocket Framing .
Acronym
Expanded
One-liner
LCP Largest Contentful Paint
Time until the biggest visible element renders; target < 2.5s.
FCP First Contentful Paint
Time until any text or image renders.
FID First Input Delay
Time from first user interaction to browser response (deprecated in favor of INP).
INP Interaction to Next Paint
Worst latency for any interaction during the page session; target < 200ms.
TBT Total Blocking Time
Sum of long-task time blocking the main thread.
CLS Cumulative Layout Shift
Sum of unexpected layout shifts; target < 0.1.
TTFB Time to First Byte
Server response delay; CDN edge helps here.
TTI Time to Interactive
Time until the page is reliably responsive.
Reliability / SRE
Acronym
Expanded
One-liner
SLA Service Level Agreement
Contractual reliability promise (customer-facing).
SLO Service Level Objective
Internal target (e.g., 99.9% availability).
SLI Service Level Indicator
Measured signal (e.g., success ratio over 5m window).
RPO Recovery Point Objective
Maximum acceptable data loss in a disaster.
RTO Recovery Time Objective
Maximum acceptable downtime in a disaster.
MTBF Mean Time Between Failures
Reliability metric; bigger is better.
MTTR Mean Time To Recover
Resolution latency; smaller is better.
DR Disaster Recovery
Cross-region failover strategy. See Multi-Region DR .
HA High Availability
Designed to survive component failures with no downtime.
Caching
Acronym
Expanded
One-liner
LRU Least Recently Used
Evict the item used longest ago.
LFU Least Frequently Used
Evict the item with the lowest access count.
FIFO First In, First Out
Evict in insertion order.
MRU Most Recently Used
Evict the most recently used item (rare; good for sequential scans).
ARC Adaptive Replacement Cache
LRU + LFU hybrid; self-tuning.
TTL Time To Live
How long a cache entry stays fresh before expiry.
Messaging
Acronym
Expanded
One-liner
AMQP Advanced Message Queuing Protocol
Used by RabbitMQ.
MQTT Message Queuing Telemetry Transport
Lightweight pub/sub; popular in IoT.
STOMP Streaming Text Oriented Messaging Protocol
Simple text-based message protocol.
DLQ Dead Letter Queue
Holding pen for messages that repeatedly fail processing.
Auth & security
Acronym
Expanded
One-liner
OAuth Open Authorization
Delegated authorization protocol. See OAuth 2.0 & OIDC .
OIDC OpenID Connect
Identity layer on top of OAuth 2.0.
JWT JSON Web Token
Signed, self-contained claims token. See JWT Internals .
SAML Security Assertion Markup Language
XML-based SSO protocol; enterprise.
SSO Single Sign-On
One login, many apps. See OAuth SSO .
MFA Multi-Factor Authentication
Two or more factors required to authenticate.
RBAC Role-Based Access Control
Permissions assigned to roles, roles assigned to users.
ABAC Attribute-Based Access Control
Policy decisions based on subject + resource attributes.
CSRF Cross-Site Request Forgery
Tricking a logged-in user's browser into making an unwanted request.
XSS Cross-Site Scripting
Injecting attacker-controlled JS into a victim's session.
CORS Cross-Origin Resource Sharing
Browser policy controlling cross-origin requests.
AI / LLM
Acronym
Expanded
One-liner
LLM Large Language Model
Transformer model trained on text corpora at scale (GPT-4, Claude, Llama).
VLM Vision-Language Model
LLM that also accepts images (GPT-4V, Claude 3 Sonnet, Gemini).
MoE Mixture of Experts
Sparse architecture: only a few expert sub-networks run per token (Mixtral, DeepSeek).
RAG Retrieval-Augmented Generation
Inject retrieved docs into the LLM prompt at query time. See RAG Patterns .
MCP Model Context Protocol
Anthropic-led open standard for connecting LLMs to tools and data sources. See MCP .
A2A Agent-to-Agent
Emerging pattern for agents to communicate over a shared protocol.
HyDE Hypothetical Document Embeddings
RAG trick: have the LLM write a hypothetical answer, embed it, then retrieve.
BM25 Best Match 25
Classical lexical ranking; the keyword half of hybrid search.
RRF Reciprocal Rank Fusion
Combine multiple ranked lists by summing 1/(k+rank).
FT Fine-Tuning
Update model weights on task-specific data.
RLHF Reinforcement Learning from Human Feedback
Train a reward model from human ratings, then fine-tune the LLM against it.
DPO Direct Preference Optimization
Simpler alternative to RLHF; trains directly on preference pairs.
LoRA Low-Rank Adaptation
Parameter-efficient fine-tuning; freezes the base, trains small adapter matrices.
PEFT Parameter-Efficient Fine-Tuning
Umbrella term for LoRA, prefix tuning, adapters, etc.
MMLU Massive Multitask Language Understanding
Standard LLM benchmark across 57 subjects.
HNSW Hierarchical Navigable Small World
The de-facto ANN index for vector search.
IVF Inverted File index
Cluster-based ANN index used by FAISS.
TTFT Time To First Token
LLM streaming latency until the first output token appears.
TPS Tokens Per Second
LLM throughput; both ingest and generation.
KV cache Key-Value cache
Per-token attention cache that makes generation amortized linear in tokens.
Infrastructure
Acronym
Expanded
One-liner
K8s Kubernetes
Container orchestration platform.
CRD Custom Resource Definition
Extend K8s with new API objects (Operators).
CRI Container Runtime Interface
K8s abstraction over runtimes (containerd, CRI-O).
CNI Container Network Interface
Plug-in interface for K8s networking.
IAM Identity & Access Management
Cloud auth/authorization (AWS IAM, GCP IAM).
VPC Virtual Private Cloud
Logically isolated cloud network.
ALB / NLB / ELB Application / Network / Elastic Load Balancer
AWS L7 / L4 / legacy load balancers.
DNS-RR DNS Round Robin
Multiple A records returned in rotation; primitive load distribution.
HPA Horizontal Pod Autoscaler
K8s autoscaler that adds/removes pods.
GSLB Global Server Load Balancing
DNS-level steering across regions.
Data engineering
Acronym
Expanded
One-liner
ETL Extract, Transform, Load
Classic batch data pipeline shape (transform before load).
ELT Extract, Load, Transform
Modern lakehouse shape (load raw first, transform in-warehouse).
CDC Change Data Capture
Stream DB row changes (see Core section).
DWH Data Warehouse
Columnar OLAP store: Snowflake, BigQuery, Redshift.
DML / DDL Data Manipulation / Data Definition Language
SQL row-level vs schema-level statements.
HLL HyperLogLog
Probabilistic cardinality estimator. See Probabilistic Data Structures .
CMS Count-Min Sketch
Probabilistic frequency estimator.
Observability
Acronym
Expanded
One-liner
APM Application Performance Monitoring
Trace, metric, error tooling (Datadog, New Relic).
OTel OpenTelemetry
Vendor-neutral observability SDK + protocol.
TSDB Time-Series Database
Optimized for append-heavy timestamped data (Prometheus, InfluxDB). See Time-Series DB .
RUM Real User Monitoring
Performance signals from real visitors, not synthetic checks.
Software lifecycle
Acronym
Expanded
One-liner
CI Continuous Integration
Auto-build and test on every commit.
CD Continuous Delivery / Deployment
Auto-promote to staging / prod.
SDLC Software Development Life Cycle
Plan → build → test → deploy → operate.
MVP Minimum Viable Product
Smallest product that delivers user value.
ROI Return On Investment
Generic value-vs-cost metric.
If you spot an acronym this list misses, open an issue or PR . The bar for inclusion: appears in at least two interview-frequency tier-1 topics on this site, or is genuinely ambiguous without expansion.
FAQ
How is this glossary organized?
By topic category, not alphabetical. Lookup is faster when you know the area you're in: open the Networking section while reading a networking page, the Database section while reading about MVCC, and so on.
Which definitions are linked to deeper pages?
Where this site has a dedicated topic page for the acronym (CAP, MVCC, OAuth, etc.), the term is linked. Definitions without links are either too primitive to deserve their own page or not in the lean topic set.
Is this the only place an acronym is defined?
No. The first occurrence of an acronym on any topic page expands it inline. This glossary is the reference of last resort and an SEO entry point for visitors searching the bare acronym.