Proposition (7)
I believe the answer is:
theorem
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">proposition</span>' is the definition.<br/ ><span class="explanation-format__explanation">(theorem is a kind of proposition)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for theorem that I've seen before include "this has been established" , "Mathematical proposition proved by reasoning" , "Mathematical statement to be proved" , "Statement to be proved in maths" , "Formula to be proved" .)