In poker, an isolation play is usually a raise designed to encourage one or more players to fold, specifically for the purpose of making the hand a one-on-one contest with a specific opponent. For example, if an opponent raises and a player suspects he is bluffing, a player may reraise to pressure other opponents to fold, with the aim of getting heads up.
Isolation plays are most common against overly-aggressive players ("maniacs") who frequently play inferior hands, or with players who may have a drawing hand. Isolation plays are also common in tournaments to isolate a player who is "short stacked", that is, one who is in imminent danger of elimination, and so is likely to be playing aggressively out of desperation.
Isolating is encouraged when holding a hand that fares better heads up than in a multi-way pot. For instance, when a player has a small pocket pair he may raise a large amount simply to knock out other players because typically a small pocket pair is about 50 percent in a heads up situation, but worse when facing multiple opponents.