Now I'm getting muddled too! Let me refresh:
(Spyder quote): In its simplest definition collection is the even loading of weight on all four legs that allows for no resistance to the riders demands and in fact the horse has given its body controls entirely up to the rider.
That might be a simple definition, but it describes such an extreme state, it would mean practically no one experiences it.
Unless you consider collection something that comes and goes. I usually think of it as Best Posture, when you're in perfect balance, and yet have the strength and readiness to jump in any direction, right then and there, whatever the rider has in mind. Strangely, the image I keep in mind is the experienced barrel racing horse, who is coiled up at the start, waiting. Once he starts, I guess the collection is lost! In any case, I think the word IS used differently in different sports.