The Collaborative International Dictionary of English v.0.48:

Cockcrow \Cock"crow\, Cockcrowing \Cock"crow`ing\, n.
   The time at which cocks first crow; the early morning; the
   first light of day.

   Syn: dawn, dawning, morning, aurora, first light, daybreak,
        break of day, break of the day, dayspring, sunrise,
        sunup.
        [1913 Webster + WordNet 1.5]

Easton's 1897 Bible Dictionary:

Cock-crowing
   In our Lord's time the Jews had adopted the Greek and Roman
   division of the night into four watches, each consisting of
   three hours, the first beginning at six o'clock in the evening
   (Luke 12:38; Matt. 14:25; Mark 6:48). But the ancient division,
   known as the first and second cock-crowing, was still retained.
   The cock usually crows several times soon after midnight (this
   is the first crowing), and again at the dawn of day (and this is
   the second crowing). Mark mentions (14:30) the two
   cock-crowings. Matthew (26:34) alludes to that only which was
   emphatically the cock-crowing, viz, the second.