The definition I've used is "wearing clothes in such a way as to imitate the opposite sex." So if a man wears a skirt, but makes no attempt to pass himself off as a woman, it isn't crossdressing.
Of course, this gets interesting when transgendered people are considered. Is it crossdressing for an MtF to present herself as female? Possibly not. Should it be "the opposite gender", not "the opposite sex?" That's a thought. But what of people who consider themselves to be both male and female, or neither, or something else entirely?
You know what? It's a useless term, I think.
Wear whatever clothes keep you warm (or cool) and make you comfortable and happy. And don't pretend to be anyone when you wear them -- in the most overused and undermeant phrase in the American language, be yourself -- whoever that may be. That's what I say. (I know it's not always that easy...It should be.

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