noun, adverb
afternoon; p.m.
Refers to the time from noon until evening. Can be used as a noun or adverbially.
会議は午後2時からです。
The meeting is from 2 p.m.
午後はいつも眠くなる。
I always get sleepy in the afternoon.
午前 means 'morning' or 'a.m.', the period before noon, while 午後 is the afternoon.
From Chinese-derived kanji 午 (noon) and 後 (after). The reading ごご is the on'yomi of both characters.