Single source of truth

A declared authoritative owner for a defined data field or record, with other representations derived or reconciled under explicit rules.

Authority can belong to different systems for different fields: a billing system may own subscription status while a profile service owns a display name. Event histories, distributed databases, and collaborative records may not reduce to one physical row. The useful contract states which owner decides, how copies update, and what happens during conflict or outage.

Builder example

An assistant needs field-level authority and freshness rules rather than a vague instruction to trust one app. Retrieval should preserve system identity, version, and timestamp so a stale cached view cannot silently outrank the owner.

Common confusion: A source of truth is an authority contract, not necessarily one file, one database, or one copy. The phrase becomes harmful when it hides multiple legitimate owners or unresolved synchronization.