Sexy, to me, is a funny term. Much like Love, it can encompass so many things. The way someone moves or acts, an article of clothing, driving fast, confidence, etc. As such, though, it makes the word hard to define. I suppose the simple definition could be "Attractive" in one sense, but it seems to fall so short.
For me, here's just a few things I find Sexy:
Behavior: confidence can be sexy, but so can shyness if done right. Definitely humor to me is sexy. If a woman can make me laugh and has a good sense of humor, that's amazingly sexy. Witty flirting falls in here too.
Activities: This will sound strange, but gaming. There is something sexy about a woman sitting down and serving up a beat down in a game. Additionally, just the simple act of laying someplace and chatting for hours about whatever topic comes up is sexy. And dancing...I suck at it, since I'm uncoordinated and can't do much more than slow dance anyway with my bad leg. However, seeing a woman dance, particularly if she's just having fun, enjoying the music...there's something very appealing.
Clothing: I won't lie here...Lingerie = Sexy. But additionally, more everyday things too. Like T-shirt and Jeans (particularly if it's a kind of nerdy shirt like game related or something from WOOT, etc) or a simple, yet fun dress. Revealing doesn't automatically make it more sexy, but won't lie that it can help. And, honestly, the sexiest thing a woman can wear is one of my shirts. I can't explain it...just something about it.
Body Parts: I won't lie here either...as a guy, I would have my maleness likely revoked if I didn't say T&A, but those actually rank behind eyes and smile for me.
Lastly, if a woman is wearing this in some form, that's super sexy: