When a person guards a "safe" or "base" spot during a game of tag, forcing the person at the base to get tagged when they come out.

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When the person who's "it" in freeze tag waits right by someone touching the base, so they can immediately be frozen when they quit touching it. The definition can be expanded to include someone in a position of power who bides his/her time to catch a 2nd person in a mistake, often for self-benefit (especially if the 2nd person is considered a nuisance).