Hard to say. Flirting is ambiguous and supposed to be difficult to pick up on by the nature of its intent (just for that one person, *wink wink* *nudge nudge*).
Without cheating by using a dictionary, my personal definition is interpersonal behavior displaying that you have the intent to start a sexual relationship at some point in the future directed at the intended recipient of the said behavior, regardless of whether it actually happens or not. Essentially it's the "pre-dating/sex" phase of a relationship.