Mathematical relationship (8)
I believe the answer is:
equation
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mathematical relationship</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)