enharmonic
Two notes that sound the same but are written differently, such as F-sharp and G-flat.
In Depth
In equal temperament (the tuning system used on modern keyboards and fretted instruments), F♯ and G♭ are acoustically identical — they produce the exact same frequency. The choice of spelling depends on the harmonic context: in a key with sharps, the note is written as F♯; in a key with flats, the same sound is written as G♭.
Enharmonic equivalence is a uniquely modern concept. In older tuning systems like meantone temperament, F♯ and G♭ were actually different pitches — F♯ was slightly lower than G♭. This distinction mattered in practice: keyboard instruments in the Baroque era sometimes had split black keys to accommodate both pitches. Equal temperament sacrificed this subtle distinction for the convenience of being able to play in any key.
On a Baroque harpsichord with split keys, there could be up to 19 notes per octave instead of the 12 we use today — each enharmonic pair had its own distinct key.