Any moment where someone does the thing you wanted them to do.
A 'conversion' is any goal action: a purchase, a signup, a download, a form submission, a call booked. What counts as a conversion depends on the funnel stage. An ad-level conversion might be 'clicked and landed on the page.' A funnel conversion is 'submitted the form.' A business conversion is 'became a paying customer.' Be specific about which.
Being sloppy about which conversion you're measuring leads to sloppy marketing decisions. Every dashboard should label the conversion event explicitly. 'Conversion rate = 2%' is meaningless without knowing the numerator and denominator.