Cell size, also know as resolution, is the size of each individual cell, or square, on an image.

Cell size, also known as resolution, is the size of each individual cell, or square, on an image. These hundreds, thousands, or millions of cells are collectively known as a raster. This is a way of quantifying information in a grid format over an image.

http://wiki.gis.com/wiki/index.php/Cellsize