Mathematical relationship (8)
I believe the answer is:
equation
'mathematical relationship' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for equation that I've seen before include "It can be algebraic" , "Mathematically, two are equal" , "E=mc2, say" , "See 5" , "E.g. 2 + 2 = 4" .)