With the first letter capitalized and always with two decimal places in the number, a term often used by Joe Monzo to delineate 1200 logarithmic divisions of the octave, thus exactly analogus to Ellis's measurement of cents.
Monzo writes: "I feel that since the prime-factor or ratio notations give precise measurements, and 1/1200th of an "octave" is approximately the limit of human pitch discrimination, more precision than this is not ordinarily needed, and I prefer to use the decimal point so that the interval may be related immediately to the familiar 12-edo scale. I use cents on occasion, when I feel that more precision is valuable."
With all letters in lower-case and no decimal places in the number, the term simply refers to a logarithmic division of 1/12 of an "octave", or one degree or "half-step" in the familiar 12edo scale. In this specific sense, the Semitone is calculated as the 12th root of 2 -- 12√2, or 2(1/12) -- an irrational proportion with the approximate ratio of 1:1.059463094359.
Successively closer rational approximations to the semitone are:
ratio prime-factorization approx. cents error from 2(1/12) 18:17 21 32 17-1 - 1.0454 (~ - 1 1/22 ) 89:84 2-2 3-1 7-1 891 + 1/10 196:185 22 5-1 72 37-1 - 1/170 1657:1564 2-2 17-1 23-1 16571 - 1/3,400 7893:7450 32 5-2 149-1 8771 - 1/86,000
(For some base-60 approximations, see Monzo, Simplified sexagesimal approximation to 12edo and Monzo, Speculations On Sumerian Tuning.)
The term "semitone" is also used loosely in a general sense to indicate any interval of approximately 100 cents -- or even more generally, approximately half of a whole-tone -- including the limma, apotome, and many others.
A Half Tone, a musical interval ranging from about 25/24 (71 cents [¢]) to 27/25 (133¢). Unless qualified by context, a semitone equals 100¢. Semitones measuring less than 100¢ are technically microtones.