Used as an adverb meaning 'late' in time, or as a noun in expressions like 遅くまで (until late).
I stayed up late last night.
He always comes home late.
早く means 'early/quickly' while 遅く means 'late/slowly', making them opposites for both timing and speed.
Adverbial form of the adjective 遅い (おそい), which is of native Japanese origin.