Definition of side bet
A side bet is a "side bet". A side bet is a bet that you make outside the poker table. It can be a bet on how the tournament will go or how you will perform in the tournament. For example, you enter a tournament with 5 of your friends. You make a "last longer" side bet. Last longer means that the player who stays in the tournament the longest will win the bet. So for example you could all put a 20??? bet into a prize pool and the player who last exits the tournament wins the prize pool.
The multitudes of possible side bets
There is no limit to the possible side bets you can make. You can bet on who will have the best hand during the night, you can side bet on who will eliminate the most players, you could bet on who after 1 hour of play or 2 hours of play will have the biggest stack. Side bets are very common among Las Vegas pros.