To give someone back the money they have spent on something — or, in the sentences people actually write, to pay a cost so that someone else does not have to carry the whole bill. In real use it almost always sits next to “the cost,” “the expense,” or “the expenses”: a grant defrays travel costs; a bake sale helps defray the price of a trip. It is formal, a little old-fashioned, and at home in letters and notices. Younger people mostly just say “pay for,” “cover,” or “pay you back.” Though actually, it does not only mean “repay after the fact.” It often means help with the bill in the first place, and it often means only part of the bill, not every last coin.