Cartesian plane divider (4)
I believe the answer is:
axis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cartesian plane divider</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for axis that I've seen before include "Imaginary line around which a body rotates" , "all revolves around this" , "from Germany and Italy, originally" , "Imaginary line around which object rotates" , "Line through the middle" .)