Each of the (three, four, or five) periods into which the night was anciently divided. Now often in collective plural, the watches of the night…

A watch, guard, or vigil kept during the night; the action of keeping such a watch; the time during which such a watch is kept.

As a measure of time. A fourth part of the night; one of the four divisions or watches of a twelve-hour night. Now historical.

One or other of the four watches into which the Romans divided the night. Obsolete.

One of the ‘watches’ or divisions of the night (Latin vigilia). Obsolete.

the time when a quarter of the night has passed (rare after 15th cent.; now poetic).

Ancient History. The last of the three or four watches into which the night was divided in the ancient Roman and Jewish worlds. See watch, n. 4.