A big bet is the larger of the two bets used in a fixed limit game, or two times the big blind in a blinds game.

In a fixed limits game this amount is the minimum bet on one of the further betting rounds. For example in 7 card stud the big bet is the minimum bet on the 5th street, whereas in hold'em the big bet is used on the turn.

In a no-limits game the minimum bet doesn't change on each round and there is actually not rule-defined big bet. It is just by convention that the big bet refers to two times the big blind. This is mainly used for comparison, especially in the BB100 rate.