enharmonic

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Of, relating to, or involving tones that are identical in pitch but are written differently according to the key in which they occur, as C sharp and D flat, for example.

from The Century Dictionary.

• In Greek music, pertaining to that genus or scale that is distinguished from the diatonic and the chromatic by the use of intervals of less than a semitone.
• In modern music: Pertaining to a scale or an instrument using smaller intervals than a semitone.
• Pertaining to a use of notes which, though differing in name and in position on the staff, refer on instruments of fixed intonation, like the pianoforte, to identical keys or tones; thus are enharmonically distinct, but practically identical.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Anc. Mus.) Of or pertaining to that one of the three kinds of musical scale (diatonic, chromatic, enharmonic) recognized by the ancient Greeks, which consisted of quarter tones and major thirds, and was regarded as the most accurate.
• adjective Pertaining to a change of notes to the eye, while, as the same keys are used, the instrument can mark no difference to the ear, as the substitution of A♭ for G♯.
• adjective Pertaining to a scale of perfect intonation which recognizes all the notes and intervals that result from the exact tuning of diatonic scales and their transposition into other keys.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective music Describing two or more identical notes that are written differently when in different keys
• adjective music Of or pertaining to a tetrachord