A secret identity is an alternate identity a figure assumes to voluntarily hide who they are. For example, the comic book character Superman's secret identity is that of Clark Kent, a mild-mannered reporter that blends into the crowd. By maintaining his secret identity, Superman, as Clark, can maintain a life outside of fighting crime and take refuge from his foes.
A secret identity is often a closely guarded secret. Many comic book story arcs center around the idea of a character's secret identity being sought after by villains or being outed, willingly or not. In Marvel's Civil War event, the character Spider-Man chose to reveal his identity as Peter Parker to the world. However, this act was undone in a later story arc that revised Spider-Man's history.