Value Bet: any bet you make that you hope will be called by a worse hand.
For example: you have A♣K♠ and the board is A♥3♦7♣ and you bet hoping to get called by a hand such as A♦10♣
1. it's not the nuts, and yet you are still hoping to get called by a worse hand.
2. if you get raised on a dry board such as the above by a very tight post flop player you might re-evaluate if you are best or behind a hand like a set. THIS DOESN'T CHANGE THE FACT THAT YOUR INITIAL BET WAS FOR VALUE.
3. being ahead of your opponents range is enough to bet for value. Often you will here terms like "making a thin value bet"-- meaning that you have only a slight advantage in equity.
4. you don't need to "know" that you are ahead: as pointed out having a term ("value bet") for "betting with the nuts" isn't very useful.