Intimacy to me
First, I agree that this is an awesome topic!
I'm not sure I can define intimacy itself in the abstract, beyond that it is an experience. I feel more comfortable listing the acts that have been intimate between myself and my partners, or between myself and other people.
I think cuddling and sex and other forms of conventional physical intimacy are always intimate for me, though I know others who might disagree. I also think sharing entertainment (watching a movie, attending a concert, visiting the aquarium, etc.) can be a form of intimacy too.
With my partner K, being read aloud and sung to are my favorite forms of intimacy. With my partner Z, showering together (we're actually monogamous shower buddies!) and being cooked for are huge. Long car trips are totally intimate in my mind for all of us (though I don't drive, so I don't know if K or Z would agree!), and in my mind sharing about oneself through conversation is probably the most intimate act of all.