Directors Joe & Anthony Russo have stated that the name of the film was not uttered by any characters in Avengers: Infinity War, but it is possible that it was said in another movie.
A Reddit user believes that this might be the case. After recently rewatching Age of Ultron, the user known as HeisenDiaN noticed a small bit of dialogue between Vision and his future squeeze Scarlet Witch, where the word 'annihilation' was said.
Other Reddit users were divided on this discovery.
This is of course pure speculation, but it might hold some truth if the leaked description of the Avengers 4 trailer turns out to be real and not just some concocted fantasy.
Although the leak didn't suggest what the title was, it did say that Vision, despite [spoilers] dying at the hands of Thanos in Infinity War ,is due to return in some capacity in the next movie.
Marvel president Kevin Feige has said that the name of the film will be revealed before the end of 2018, as they begin to promote the release of Captain Marvel more heavily.
Avengers 4 will be released in May 2019 and is expected to be the final outing for a few Marvel mainstays, including Chris 'Captain America' Evans who has confirmed he is stepping away from the role.