Enharmonic

In music, two notes, intervals, or key signatures are enharmonic if they sound the same but are written in different ways. For example, C-sharp and D-flat are enharmonic notes. C-sharp major and D-flat major start on enharmonic notes, so they are enharmonic key signatures, but they have different numbers of sharps and flats. An augmented fourth and a diminished fifth are enharmonic intervals, because they both cover 6 half steps.