When is a plant technically in flower?

Normally when you notice the first pistils will she be in flower this plant is very close she is pre flowering telling you she's ready to be flipped :) Now if your asking when you should start counting down them pesky 8 weeks these breeders always bang on about. I usually advise starting the 8 week mark when you see the first actual bud. That way you will get a much clearer harvest time breeders exaggerate just as much as men do Im afraid so they very rarely finish in the time state because you need "Optimal conditions" whatever they are lol.