Secret identity
In works of fiction, especially comic books about superheroes, the hero will normally have an alias and costume aside from their real name while keeping the fact they’re superheroes a secret from everyone in civilian life. For example, Spider-Man doesn’t tell people he’s actually Peter Parker in a mask because then supervillains would try to kill the people Peter knows to get to him. And that’s also why Batman doesn’t tell people he’s Bruce Wayne and Superman doesn’t tell people he’s Clark Kent. Some supervillains will also have a secret identity because they don’t want people to know that they’re criminals. There are also some superheroes who don’t have a secret identity.