Definition of double up
A double up is when you double up your stack on a hand. This is usually referred to as a double up in tournaments. A cash game player who doubled up on a hand would have stacked his opponent more than doubled up. Double up is crucial and common in tournaments. When we have more than 10 big blinds in a tournament, we will be looking for a double up situation at all costs, i.e. pushing our stack with a decent hand (if it is strong, it will be a plus) and hoping to double up. If we lose the hand, we're out, but if we double up, we're back in a good position in the tournament.
Use of the expression double up
The term double up is almost always used in tournament play, and often in pre-flop all-ins. In cash games, the payouts can be more gradual. You can go from 100 big blinds to 200 big blinds with several small prize pools. In tournament play, since we often have less than 100 big blinds, we have more opportunities to double up. If two players with the same stack are all in preflop, one will get double up and the other will get stacked!