A defined event names a meaningful action in terms of raw events you already collect — "signed up" meaning a particular submit on a particular page.
Why they matter
They are retroactive. Define one today and it applies to history you already collected, because it is evaluated when you query rather than being written into the data.
That has two consequences worth knowing:
- You do not need perfect instrumentation up front. Capture broadly, name things later.
- Changing a definition changes past reports too. That is usually what you want — the definition was wrong — but it means a number can move without new data arriving.
Never materialised
Definitions are not stored as new events. There is no backfill step and no duplicate data; the definition is applied at query time.
Creating one
Build it from the events and properties that identify the action, then use it anywhere a normal event can be used — funnels, retention, cohorts.