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witness [ noun ] [ noun ]

Definition​

a witness is an entity or component designated (trusted) by the controller of an identifier. The primary role of a witness is to verify, sign, and keep events associated with an identifier. A witness is the controller of its own self-referential identifier which may or may not be the same as the identifier to which it is a witness. See also KERI’s Algorithm for Witness Agreement.
Source: Dr. S. Smith

In KERI and ACDC context, a witness is an entity or component designated (trusted) by the controller of an identifier. The primary role of a witness is to verify, sign, and keep events associated with an identifier. A witness is the controller of its own self-referential identifier which may or may not be the same as the identifier to which it is a witness.

An identifier witness, therefore, is part of its trust basis and may be controlled (but not necessarily so) by its controller. The purpose of a pool of witnesses is to protect the controller from external exploit of its identifier.
The term Backer and Witness are closely related in KERI but not synonyms or interchangeable.

KERI witness confusing​

Be sure to understand the narrow KERI definition of Witness well. You could easily be confused, for there are dozens of papers that use the term Witness in a similar way to KERI; for example https://ieeexplore.ieee.org/document/8644609 or 'segregated witness' in bitcoin, but it's far from the same concept.
More in the whitepaper

Operational description in KERI​

Entity that may receive, verify, and store key events for an identifier. Each witness controls its own identifier used to sign key event messages, a controller is a special case of a witness.
Source Sam Smith

Other glossaries (or mental models)

witness:

A computer system that receives, verifies, and stores proofs of key events for a verifiable identifier (especially an autonomous identifier). Each witness controls its own verifiable identifier used to sign key event messages stored by the witness. A witness may use any suitable computer system or database architecture, including a file, centralized database, distributed database, distributed ledger, or blockchain.

Note: KERI is an example of a key management system that uses witnesses.

(source)

witness:

An appropriately cleared (if applicable) and designated individual, other than the COMSEC Account Manager, who observes and testifies to the inventory or destruction of COMSEC material.(source)

witness:

An entity that is designated by the controller of an AID to verify, sign, and keep the key events associated with the AID, as defined by the KERI whitepaper. See WebOfTrust glossary for more detail.(source)
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