To call off something means to cancel it or decide it won't happen after all. When the principal calls off school because of a snowstorm, she's announcing that classes are canceled for the day. When a baseball game gets called off due to rain, the umpires decide the teams won't play that day.

The phrase often implies that something was already planned or in progress. Your parents might call off a picnic when dark clouds roll in, or a search party might be called off once a lost hiker is found safe. Sometimes people call off a wedding if they realize they shouldn't get married, or call off a deal if the terms aren't right.

You can also call off a dog or another person who's pursuing something, telling them to stop. If your dog is chasing a squirrel and you shout for them to stop, you're calling them off. In old westerns, one character might tell another to “call off your men” when asking them to stop an attack or pursuit.

The phrase carries a sense of authority: someone with the power to make decisions is stopping something that was supposed to happen. It's more definitive than postponing or delaying, which means pushing something to a later time. When something is called off, it's done, at least for now.