Second Brain / Industry term
Single source of truth
The one place you treat as authoritative for a given fact, so every other copy is read as a view of it rather than a competing version you have to reconcile.
A single source of truth is the one place you treat as authoritative for a given fact, so every other copy is read as a view of it rather than a competing version you have to reconcile. When the fact changes, you change it in that one place and let the copies follow. Picture a to-do app where the same task is written in a notes file, a calendar, and a chat message. With no source of truth, three versions drift apart and you no longer know which due date is correct. Name one record as authoritative, and the others become reflections you can regenerate instead of arguments you have to settle.
Builder example
When you point an AI assistant at your records, the source of truth decides which answer it trusts. If a contact's email lives in three places with two spellings, the assistant may draft a password reset to the wrong address and sound confident doing it. Tell the assistant which record is authoritative and have it read the others as copies, so a stale duplicate never overrides the fact you maintain.
Common confusion: A single source of truth is about authority, not about storing a fact only once. Copies are fine and often useful. What matters is that one record decides the answer and the rest defer to it when they disagree.