A transformation sequence is any series of images that show a character's transformation from one thing to another. Usually it refers to a trope in anime or manga where a character transforms into a superpowered alter ego. There are examples in live action film and television, but they are few and far between.
Transformation sequences occur frequently in anime and manga, but they also appear in Western superhero fiction. They are characterized by the ritualized changing of a character, from the mundane to the fantastic. Elements of their costume may change, their hair may change in length or color and they may acquire weapons or other symbols of their new identity.
In the Sailor Moon anime, Usaji's sailor costume grows out of her compact mirror. Her gloves form from ribbons of light, and her headband grows out of her forehead. This transformation sequence occurs at least once an episode. Western examples of the transformation sequence include Diana Prince spinning into Wonder Woman on the live action television show, as well as Clark Kent's transformation into Superman. In both the television show and the Fleischer & Famous Superman cartoons from the 1940s, Clark Kent dashes into a telephone booth and almost instantaneously emerges as Superman.