The way I was taught in school is: the abscissa (horizontal) and the ordinate (vertical), but it might sound to pompous for some people. I would suggest calling them the horizontal and the vertical axes, but never the "x and y", because the name of the variable is irrelevant.
Thank you all for the answers; I understand now that is more import is to use terms significant in the specific context.
Since I am referring to points in the cartographic domain, I should probably be using terms like "easting axis" and "northing axis" or "m axis" and "p axis".