Conversion event
In GA4, a conversion event is an event flagged as a goal: purchase, form submission, newsletter signup, or download. It replaces the 'goals' of the previous Universal Analytics and supports more flexible, event-based measurement across the user journey.
In practice
GA4 tracks purchase, generate_lead, and a few e-commerce events by default. Custom conversions (clicking a phone number, watching 50% of a video, scrolling to a key section) are configured in GTM or directly in code with gtag('event'). Each conversion should carry an estimated monetary value to activate ROAS columns in reports and enable value-based bidding strategies in Google Ads.