How about axis?
- (Mathematics) one of two or three reference lines used in coordinate geometry to locate a point in a plane or in space.
(Collins English Dictionary – Complete and Unabridged)
Update:
You say you're looking for a name for a code variable. Seeing as people here struggle so much to find such a word, I'm guessing whatever you settle on will confuse whoever reads your code just as much. I'd just skip trying to be pithy and go for rowOrColumn. Clarity over brevity.