Redefining variables used in many equations in a publication
I think of this as a matter of "breadcrumbs."  If a reader is constantly wondering what "a" is and having to look back to try to find its definition, that is a problem.  On the other hand, if I put so much redundant definition in that the math gets hidden by uninteresting boilerplate prose, that's also a problem.

My preference is thus to take a two-pronged approach:


Create a table of symbols that collects all of the definitions in one easy-to-find location, and
Match every equation with a prose summary of its meaning.


For example, if I were presenting Newton's laws, then I might say:


  "The force exerted by an object is proportional to mass and acceleration: F = ma."


and in the table of definitions "F" and "m" and "a" would all have their formal definitions.  In this way, I remind the reader about the definitions without repeating them, yet at the same time have a simple reference point at which all definitions may be readily found.