Mathematical statement of equivalence (8)
I believe the answer is:
equation
'mathematical statement of equivalence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for equation that I've seen before include "Ambrose --, author of The Devil's Dictionary" , "Maths statement with =" , "Mathematical statement of equivalence" , "See 5" , "E.g. 2 + 2 = 4" .)