Two-dimensional
I believe the answer is:
planar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">two-dimensional</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for planar that I've seen before include "Lying in a flat surface" , "In two dimensions" , "12 [TWO DIMENSIONAL]" , "Ran lap (anag) - flat" .)