gap year什么意思

It was your 50th birthday, Mr Welsborough, of course you were disappointed that your son hadn't made it back from his gap year. After all, he was in Tibet. Yes. No. No?

那天是你50岁生日，韦尔斯伯鲁先生，你当然非常失望你儿子还在毕业旅行中没赶回来，他人可在西藏呢。对。不对。不在吗？

—《神探夏洛克 第四季》