When setting out an equation, it is common to have a block of text that details the notation used, assumptions made and conditions under which the equation is valid.
For example, Newton's Law of Gravitation needs some qualifiers if you want to use it to tell you the force on an object due to another's gravity:
F = (G M m) / d 2
- F = attractive force
- G = gravitational constant
- M, m = mass of two objects
- d = separation of the objects' centres of mass
This only applies when the objects are not subject to relativistic effects and we assume no other forces act on the objects.
The word I'm looking for describes the text from "where:" to "objects". I had considered rubric, which means more "special instructions", but I don't believe this applies to descriptive, rather than instructional, text.