Kind of equation (9)
I believe the answer is:
quadratic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">kind of equation</span>' is the definition.<br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for quadratic that I've seen before include "Second-power equation" , "Type of algebraic equation" , "Mathematical expression containing a variable raised to the second power" , "(In maths) a type of equation" , "(In algebra) kind of equation - resembling a square" .)