In 2364, Deanna Troi found an entry for the Aegis-class in the USS Enterprise-D computer while searching for information regarding the ship crew with whom she had briefly made telepathic contact. (TNG novel: Ghost Ship)
There was never an Aegis class vessel. The ship Troi refers to is actually a Ticonderoga class cruiser, which was sometimes referred to as "Aegis class" in references of the time because it was the only warship then equipped with the Aegis combat system (it would be joined in 1991 by the Aegis-equipped Arleigh Burke class destroyer, and by the 21st century by half a dozen other classes of frigates and destroyers worldwide using the same or similar combat systems). The Aegis combat system is an automated fire control system for anti-air warfare.