| Actor | Fundamental unit of computation. Isolated entity that processes messages sequentially and communicates via message passing. |
| ActorSystem | Top-level runtime hosting actors. Manages lifecycle, messaging, clustering, remoting. See Actor System. |
| PID | Process identifier. Live handle to a running actor. Used for Tell, Ask, and all interactions. |
| Path | Canonical location of an actor: goakt://system@host:port/path/to/actor. Returned by pid.Path(). |
| Grain | Virtual actor. Identity-addressed; framework manages activation and passivation. |
| Mailbox | Per-actor message queue. Messages enqueued by senders, dequeued by the processing goroutine. |
| Guardian | Built-in actor serving as root of the actor tree. Root, system, and user guardians. |
| Supervisor | Parent’s failure-handling policy. Defines resume, restart, stop, or escalate. |
| Directive | Specific supervision action: Resume, Restart, Stop, Escalate. |
| Passivation | Automatic shutdown of actors to reclaim resources. Two types: per-actor (idle-based) and system eviction (capacity-based). See Passivation. |
| System eviction | Node-wide limit on active actors. When exceeded, passivates by LRU, LFU, or MRU. Configured via WithEvictionStrategy. |
| Reentrancy | Ability to process other messages while awaiting an async Ask response. |
| Stash | Side buffer where an actor can park messages for later processing. Requires WithStashing(). See Stashing. |
| Behavior | A function func(ctx *ReceiveContext) that handles messages. Actors can switch behaviors via Become/UnBecome. See Behaviors. |
| PipeTo | Runs a task asynchronously and delivers the result to a target actor. See PipeTo. |
| TestKit | Test helpers: throwaway actor system, Probe, GrainProbe. See TestKit. |
| Dead Letter | Message sent to a non-existent or stopped actor. Captured by the dead-letter actor. |
| Death Watch | Subscription to an actor’s termination; receive Terminated when it stops. |
| Cluster Singleton | Actor with exactly one instance across the cluster. Hosted on the coordinator (oldest node); relocated when that node leaves. See Singletons. |
| Relocation | Automatic migration of actors and grains from a departed node to remaining cluster nodes. See Relocation. |
| Discovery Provider | Pluggable backend for finding peer nodes (Consul, etcd, K8s, NATS, mDNS, static). |
| Extension | System-wide plugin registered at ActorSystem creation. Accessible via ctx.Extension(id). |
| Dependency | Per-actor resource injected at spawn. Must be serializable for relocation. |
| Event Stream | In-process mechanism for system and cluster events (ActorStarted, NodeJoined, Deadletter, etc.). Subscribers use Subscribe() and Iterator(). See Event Streams. |
| PubSub | Application-level topic-based messaging. Topic actor manages Subscribe/Unsubscribe/Publish. Requires WithPubSub() or cluster. See PubSub. |
| Client | Standalone package for interacting with a GoAkt cluster from outside the actor system (CLI, API server, batch job). Tell, Ask, Spawn, grains. See Client. |