A person who is active late at night.
'Nightowl' was originally just a synonym for 'owl' and has been used as such since at least 1581, when Bell and Foxe included it it their translated work Against Jerome Osorius. That seems rather tautological as owls are predominantly nocturnal and, in an apparent general acceptance of that view, the literal use of the word is now rather rare.
The figurative use of the term, that is, as a reference to people rather than owls, also began in the 16th century. Shakespeare used it in 1594 in the narrative poem The Rape of Lucrece:
This said, his guilty hand pluck'd up the latch,
And with his knee the door he opens wide.
The dove sleeps fast that this night-owl will catch:
Thus treason works ere traitors be espied.
The Bard didn't give up on the literal usage though. It appears, in contexts which make the literal reference to a bird clear, in both Richard II, 1593:
"For nightowles shreeke, where mounting larkes should sing."
and in Twelfth Night, 1602:
"Shall wee rowze the night-Owle in a Catch?"
See other phrases and sayings from Shakespeare.