Key event
An event marked in GA4 as one that matters, which is the analytics counterpart of a conversion.
What makes it wrong.
The mechanism, not the warning.
The mark is not retroactive in the advertising sense: a newly marked event will be reported historically in analytics, but platforms that imported the old definition keep optimising toward what they were given. The two systems disagree until both are updated.
In detail.
Marking an event as key changes how it is reported, not how it is collected. The event was already being recorded; the mark decides whether it appears as a result in reporting and can be imported into advertising platforms for optimisation.
The renaming from conversion to key event was made precisely to separate the analytics definition from the advertising one, because the two count differently.
Where this is measured.
Key event comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.