apotome

noun3

1. •
   The difference between two quantities or lengths commensurable only in power, as between 1 and the square root of 2, or between the diagonal and side of a square.
2. •
   The remaining part of a whole tone after a minor second has been deducted from it; an augmented unison. Most commonly used to refer to the Pythagorean chromatic semitone, which has a ratio of 2187/2048.
3. •
   A distinct division of an insect which is divided from the other divisions by a pinch point.

   • The bands pass inside the margins of the apotome as they approach its constriction and form two pigmented areas broadly based on the frontal sutures and becoming dispersed towards the mid-line. — 1964