Period of three months (9)
I believe the answer is:
trimester
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">period of three months</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 trimester that I've seen before include "Period of three months" , "part of college year" , "US academic term" , "academic session" , "American term" .)