apotome

from The Century Dictionary.

• noun In mathematics, a term used by Euclid to denote a straight line which is the difference between two straight lines that are rational (in Euclid's sense, that is, are either commensurable with the unit line, or have their squares commensurable with the square on the unit line) and that are commensurable in power only (that is, have their squares commensurable, but are themselves incommensurable).
• noun In the Pythagorean musical system, the greater of the two half steps or semitones into which the whole step or whole tone is divided. Its vibration-ratio is 2187/2048.

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

• noun (Math.) The difference between two quantities commensurable only in power, as between √2 and 1, or between the diagonal and side of a square.
• noun (Mus), obsolete The remaining part of a whole tone after a smaller semitone has been deducted from it; a major semitone.

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

• noun mathematics The difference between two quantities commensurable only in power, as between 1 and the square root of 2, or between the diagonal and side of a square.
• noun music The remaining part of a whole tone after a smaller semitone has been deducted from it; a major semitone.