Used as an adverb meaning 'late' in time, or as a noun in expressions like 遅くまで (until late).
昨夜は遅くまで起きていた。
I stayed up late last night.
彼はいつも遅くに帰宅する。
He always comes home late.
遅く means 'late' or 'slowly', the opposite of いち早く's sense of being quick or ahead of others.
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.