Definitions for "Big Blind"
The larger of the two forced preflop bets. The player to the left of the small blind posts the big blind, the blinds ensure there is always money in the pot and "action" on every hand.
The second bet placed before any cards are dealt – normally double the small blind and equal to the lower bet applying to the table. Ie in a $1/$2 game the small blind is normally 0.50$ and the big blind $1. The blinds are placed so everyone has something to play for. If you join a game part the way through you will also be asked to place a big blind.
Larger of two forced bets to ensure action on the hand. Usually equal to the value of the pre-flop minimum bet.