key event log

Definition

A verifiable data structure that is a backward and forward chained, signed, append-only log of key events for an AID. The first entry in a KEL MUST be the one and only Inception Event of that AID.
Source Sam Smith

Put differently

KELs are hash-chained Key Events. These are blockchains in a narrow definition, but not in the sense of ordering (not ordered) or global consensus mechanisms (which is not needed). (SamMSmith)

A KEL is KERI's VDS: the proof of key state of its identifier.