后天(hòu tiān) - the day after tomorrow in Mandarin

后天 (hòu tiān) is 后 (hòu, "behind, after") plus 天 (tiān, "day"): the day that sits two steps ahead of today, i.e. the day after tomorrow. Like its mirror image 前天 (qián tiān, "the day before yesterday"), it names one fixed day rather than a general stretch of future time.

It sits at the front of a sentence or right after the subject with no preposition: 后天见 (hòu tiān jiàn), "see you the day after tomorrow". Be careful not to say 后天 (hòu tiān) when you mean 以后 (yǐhòu, "from now on, afterwards") - the first is one calendar day, the second is an open-ended stretch of future time.

One quirk worth flagging: 后天 (hòu tiān) also has a completely different, adjectival use in the phrase 后天性 (hòutiānxìng), "acquired", as opposed to innate - the opposite of 先天 (xiāntiān, "innate, congenital"). That sense turns up in medical and philosophical writing rather than daily chat, but it explains why a dictionary search on 后天 (hòu tiān) sometimes returns an unexpected result.