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.

To give someone back the money they have spent on something把某人花在某件事上的錢還給他 — or, in the sentences people actually write,不過在大家實際寫的句子裡，to pay a cost so that someone else does not have to carry the whole bill通常是指付一筆費用，讓別人不用扛下全部的帳單。In real use實際使用時，it almost always sits next to這個字幾乎都會跟著 "the cost," "the expense," or "the expenses"「cost（費用）」、「expense（花費）」，或「expenses（各種花費）」這類字：a grant defrays travel costs一筆補助金defrays（支付了）旅費；a bake sale helps defray the price of a trip一場義賣幫忙defray（分擔）了旅行的花費。

It is formal, a little old-fashioned, and at home in letters and notices.

It is formal, a little old-fashioned這個字很正式，也有點老派，and at home in letters and notices常出現在書信和公告裡。

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.

Younger people mostly just say年輕人大多只會說 "pay for," "cover," or "pay you back."「pay for（付錢）」、「cover（付掉）」，或「pay you back（把錢還你）」。Though actually不過其實，it does not only mean "repay after the fact."它不是只有「事後把錢還回去」的意思。It often means help with the bill in the first place它常常是指一開始就幫忙付這筆帳，and it often means only part of the bill, not every last coin而且通常只付一部分，不是全部的錢。