(geometry) plane · flat surface · flat; two-dimensional

That Space, as our mathematicians have it, is spoken of as having three dimensions, which one may call Length, Breadth, and Thickness, and is always definable by reference to three planes, each at right angles to the others.

First of all, the level of the ocean could rise, because ocean water expands as it grows warmer.

Melting polar icecaps could also contribute to an increase in sea levels.