The nexus comma, otherwise known as the nexisma, or – in the earliest materials where this comma is named outside of color notation – the nexuma, is an 11-limit (also 2.3.11 subgroup) unnoticeable comma with a ratio of 1771561/1769472 and a value of approximately 2 cents. In color notation, this same comma is referred to as the tribilo comma. It is the amount by which a stack of three 128/121 Alpharabian diatonic semitones falls short of a 32/27 minor third, or equivalently stated, the amount by which a stack of three 121/96's exceeds the octave. It is also the difference between the rastma and the Alpharabian comma, or the sum of the schisma and the parimo.
It leads to the joining of the 11-limit and the 3-limit, a fact which, in light of the importance of both p-limits, led to Aura considering the temperament that tempers out this comma to be some sort of "nexus temperament", which it turn gave rise to most of this comma's names. While the importance of the 3-limit is generally accepted (see Pythagorean tuning, circle of fifths, FJS, Helmholtz-Ellis notation), it can be derived mathematically that the 11-limit is an excellent basis for quartertones in terms of ratio simplicity, and the 11-limit can be shown to host a clear sequence of intervals in which every other member is the octave complement of what is effectively a stack of 128/121 diatonic semitones (see Alpharabian tuning).
For a list of temperaments of various ranks that temper out the nexus comma, see the Nexus.