50¢ above a C natural. Equal to a Db, but one must know when to use the C

50¢ above a C natural. Equal to a Db, but one must know when to use the C#, and when to use the Db. It is usually dependent on the key in which you are playing, or the chord you are in.

More simply, it is the black key to the right of middle C, on the piano.