Definition of liability
In poker, when we talk about a passive player, we are talking about a player who generally does passive actions rather than aggressive ones. Folding, limping, folding and calling are passive actions. Betting, raising, 3bet and 4bet are aggressive actions. A passive player is a player who will mostly do passive actions. An aggressive player might check-raise on the flop. The passive player is more likely to check-call his draw. The only time the passive player will raise is when he has the nuts.
How do you identify the passive gambler?
If you have a tracker like Poker Tracker 4 or Xeester, you will be able to display a statistic called "Aggression Factor" in your HUD. This is usually a good indicator of how aggressive a player is. The statistic will make a ratio between the player's passive and aggressive plays and will indicate an aggression rating. To identify aggressive players, know that 2.0 of "aggression factor" is the 0 degree of aggression. At 2.0 you are "normal" aggressive. At more than 2, you are considered aggressive and at less than 2, you are considered passive. Sometimes you can see players with aggression indicators of 0.7. Against such a passive player, give all possible credit to his aggressive games!