Optical (6)
I believe the answer is:
visual
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">optical</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for visual that I've seen before include "Of the eyes or sight" , "Pertaining to sight" , "Relating to, or using, sight" , "Of or used in seeing" , "Of sight and seeing" .)