A conversion is any action you've defined as valuable and told your measurement to count — a form submit, a call, a purchase, a booking. It's not a Google term of art so much as YOUR definition, which is exactly why it goes wrong.
The classic failure is counting cheap actions (pageviews, button clicks, time-on-site) as conversions, then optimizing ads toward them: the algorithm happily buys you thousands of worthless 'conversions'.
Our rule: count as conversions only events where a human raised a hand — a submitted form that reached your inbox, a call over 30 seconds, a booked slot. Everything else is diagnostics, not success.