Definition
What is Key Event (GA4)?
A key event is the Google Analytics 4 term for an event you have flagged as especially important, such as a signup, purchase, or form submission. Key events replaced the older label conversions inside GA4 reporting. You mark any collected event as a key event in the admin settings, and GA4 then reports counts and rates for it.
Why it matters
Key events are how GA4 measures the outcomes that matter, so an action that is never marked as a key event is effectively invisible in your conversion reporting.