What is the
abomination of desolation?

What Matthew and Mark called the abomination of desolation from Daniel (Matt 24:15; Mark 13:14), Luke plainly explained were armies that would come to desolate Jerusalem (Luke 21:20). The Roman armies came during that generation and fulfilled the prophecy perfectly just as Jesus carefully explained (Matt 22:1-7; 24:32-35; Luke 19:41-44; Acts 2:40; I Thess 2:14-16).

See Also

More details: What is the “Abomination of Desolation”?
Detailed sermons and outlines: The Witness of 70 AD.