Thayer's Greek Lexicon

 1. guard, watch
  1) a watching, keeping watch
1a1. to keep watch
  2) persons keeping watch, a guard, sentinels
  3) of the place where captives are kept, a prison
  4) of the time (of night) during which guard was kept, a watch
i.e. a period of time during which part of the guard was on
duty, and at the end of which others relieved them. As the
earlier Greeks divided the night commonly into three parts,
so, previous to the exile, the Israelites also had three
watches in a night; subsequently, however, after they became
subject to the Romans, they adopted the Roman custom of
dividing the night into four watches