Enharmonic

 
     Enharmonic notes are notes that sound the same but are spelled differently. An example would be C flat and B natural. Since there is no black note on the piano keyboard going down a half step from C would put you on the same key as B natural. Another example is C sharp and D flat. There is only the one black key between the two white keys, so going up a half step from C puts you on the same key as going down a half step from D.