WARNING! SPOILERS ARE SWIMMING AROUND WITHIN! WARNING!
In the last Justice League movie subtitled, War, the movie ended with the heroes tentatively considering forming some kind of “league” to better defend the world. In this movie they finally come up with their destined name, Justice League, as we learn from Steve Trevor from a phone call while on his way to their S.T.A.R. Lab’s headquarters. Justice League tested better he tells whomever’s on the other end of the line.