Kind of equation (9)
I believe the answer is:
quadratic
'kind of equation' is the definition.
This is the entire clue.
(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" .)