to become ill or start behaving badly again, after making an improvement

She managed to stop using drugs for a month, but then relapsed. 她坚持了1个月没有吸毒，但是接着又故态复萌。

He looked happy for a brief while, before relapsing into silent misery. 有那么一阵子他看上去很幸福，但接着又陷入了无言的苦恼中。

If someone who is getting better after an illness has a relapse, they become ill again.

She was looking quite healthy on Friday, but she had/suffered a relapse over the weekend and was taken back into hospital. 周五的时候她看上去还很健康，但到了周末病情复发，又被送回了医院。