As it turns out, Doctor Strange revealed it to us near the end of Infinity War.
Marvel will released Avengers: Endgame on April 26, 2019.
Much was made of the title ahead of time and the fact that Marvel didn’t want to reveal it.
Before turning into a pile of dust as a result of “The Decimation,” aka Thanos’ extremely deadly finger snap, Doctor Strange told Tony that they were in the “endgame.”