A move may also be called sente because of the local conditions (sente position), even if the opponent does not answer it. This is theoretical rather than practical: after all one can only say what ought to be answered, not what will be.
The opposite of sente is gote.
Dieter: The above comment may account for a good definition of sente: A move is sente if the value of the follow-up move (when the move is not answered) is larger than the value of the move itself (when the move is answered).
Of course this definition assumes we know the value of a move (deiri counting). In yose (endgame) we can calculate it (miai counting). It also assumes that for each move there is another move of appoximately the same value.
Jan: I have also seen the word 'sente' used as a noun - a certain move is Black's sente. I'm still not entirely clear on what that means, and (more importantly) why that implies that Black will get that move before White - is that because of the proverb 'Sente is worth twice as much gote'?
Bill: Sente is a noun.
If a play is considered Black's sente (see sente position) it carries a threat that is larger than its reverse sente. When the ambient temperature is cooler than the threat but hotter than the reverse sente, Black will normally be able to play the sente and get a response before White can afford to play the reverse sente. That's why Black owns his sente.
Funkybside: During game 6 of the 59th Honinbo, 13k* enfo recalled an unknown 6d's definition of sente. Sente is a move you hope your opponent won't respond to. Of course this isn't helpful to a beginner, and totally ignores the concept of gote- but it sure is an interesting and realistic idea. One that seems to describe actual play as opposed to some 'ideal' concept.
ProtoDeuteric- Is there such thing as "sente-gote" or "gote-sente" (as a term), where your opponent plays some sente move, and your response (gote?) must in turn be responded to?
Niklaus: This is referred to just as a gote move by the one who played first. See for example the diagram on the gote page.
Bill: But see Sente no Gote.
Bob McGuigan: I'm familiar with the term sente no gote but, strictly speaking, isn't it really just a sente move? The sente no gote page says the move looks like gote, but that doesn't make it gote.
Bill: Perhaps ProtoDeuteric had a reversal in mind, like hane-tsugi. The first move carries a large threat, and must be answered, but the reply must be answered, too.
Bob McGuigan: I got myself back to front on this, I see. I meant to say that gote no sente should just be called sente since the move requires an answer and only looks like a gote move.
ProtoDeuteric: Bill, you've hit the nail right on the head. That is exactly what I meant.