adverb, noun
late at night; at a late hour
Refers to the late hours of the night, often used adverbially or as a noun.
夜遅くまで勉強した。
I studied until late at night.
夜遅くに帰宅した。
I came home late at night.
深夜 is a more formal noun meaning 'midnight' or 'late night', while 夜遅く is a common adverbial expression for 'late at night'.
Compound of 夜 (よる, 'night') and 遅く (おそく, adverbial form of 遅い 'late').