In horse racing, offtrack (also written off-track) means away from the racecourse itself. The horses still run at the track; you are somewhere else. The phrase you will meet most is off-track betting: you put money on a race from a shop, a bar, or a special office, without standing beside the track. In the United States those places are often called OTB parlours. Papers also use it as a plain adjective — an offtrack location, offtrack wagering. It is everyday racing-and-gambling English, not a fancy word. Outside that world, people hardly use it this way. Though actually, English has a lookalike idiom, off track, that is a different picture altogether: a plan or a conversation that has wandered (“the meeting went off track”), like a train leaving its rails. That is not this entry.