Where to define a symbol that also has an abbreviation I have a symbol &mu;, which is the linear attenuation coefficient for a material (X-ray tomography). I've already defined that symbol in my document's list of symbols, but the problem is that I also refer to it by its abbreviation, LAC, in my text (LAC is well known in the field, hence I prefer keeping it, only using &mu; in equations).

Should I add that abbreviation to the list of abbreviations, resulting in their being two entries for linear attenuation coefficient, one in the list of symbols and one in the list of abbreviations, or should I just leave it in the list of symbols?
If you actually have significant explanation beyond just expanding the abbreviation, I recommend putting that in the list of abbreviations and using a cross-reference from the list of symbols.  Just like an index can have &quot;attenuation coefficient, linear -- see linear attenuation coefficient&quot;, your lists of abbreviations and symbols could have, respectively,

LAC, linear attenuation coefficient, symbol \mu.  The linear attenuation coefficient applies to materials where absorbance dominates scattering as an attenuation mechanism, and total transmission energy follows Lambert's Law.
\mu, symbol for linear attenuation coefficient.  See also \hyperlink{LAC}

If there's no additional explanation, then both entries can be complete and short and a hyperlink would be pointless.

LAC, linear attenuation coefficient, symbol \mu
\mu, linear attenuation coefficient, abbreviated LAC