CRDT
Conflict-free Replicated Data Type — data structure where multiple replicas can be updated independently and always converge to the same final state without coordination.
Two big families: state-based (CvRDT, replicas exchange whole state and merge via a commutative op) and operation-based (CmRDT, replicas exchange operations in a causality-preserving order). Classic building blocks: G-Counter, OR-Set, LWW-Register, RGA-text.
Powers Figma, Linear, Apple Notes, Riak, Redis CRDTs and every darling of the "local-first" movement (Automerge, Yjs). Trade-off: tombstones and metadata grow unboundedly without periodic garbage collection.