A Greek word which means literally "color". It is generally used in music to indicate the perceived sensation of pitch disregarding the note's octave registration -- i.e., the tuning has an equivalence-interval, which by definition is ignored in determining the chroma.
(Greek: "color", "colored", "chromatic")
The genus with a minor third and two semitones.
Neo-Chromatic is used to describe the permutation(s) of the intervals which place the minor third in the middle of the tetrachord. Such tetrachords are found in
Before August 2004, Gene Ward Smith was using "chroma" with his own definition:
A "chroma" for an n-note chain scale of a temperament is an interval which when wedged with (the complement of) a linear temperament wedgie gives (complement again) a val v such that v=+-n, which is an n-equal val supported by the temperament. So it's only a chroma relative to a specific size of chain (MOS, without the DE assumption,) and more precisely only relative to a particular val.
Another way to say it is that the chroma is mapped to the difference between the start and end of the chain by the temperament. 25/24 would be a chroma for 7 notes of meantone, but so would 78125000/78121827. Chromas which shift concordances, particularly the generator (presuming it is concordant), to some other concordance, are particularly interesting from the point of view of circulation.
For example, 40/39 is a chroma for the 13-limit meantone from the standard vals for 31 and 50. The val you get wedging it with the wedgie is <12 19 28 34 41 45|, which is one version but hardly the only one for 12-et in the 13-limit. A nice thing about it is that (3/2)*(40/39) = 20/13, a 13-limit concordance; another nice thing is that 36/35 is a chroma with the exact same val, and (5/4)*(36/35)=9/7.