What is a Fat Quarter

A piece of fabric approximately 20″ x 22″ (50 cm x 55 cm).

A fat quarter (bottom left) compared with a fat eighth (bottom right)

The fat quarter is the most popular pre-cut fabric size for Patchwork and Quilting, due
 to its reputation as the most manageable and versatile size.  Fat quarters are a size used
 for many projects and allows quilters to build us a stash of many fabrics in various colours
 and designs without having to purchase more fabric than they may need.