Half a fat quarter
A fat quarter is about 18 × 21 or 22 inches, depending on the fabric’s width. Cut it in half across the 18 inch side and each half is a fat eighth, about 9 × 21 or 22 inches. Shops sell them singly and in bundles.
Fat eighth or eighth of a yard?
At a 42-inch fabric width, both contain about 189 square inches before trimming; at other widths the area changes. Their shapes differ. An eighth of a yard cut from the bolt is a strip 4½ inches deep across the whole width of fabric, about 4½ × 42 inches. The fat eighth is squarer, so it fits wider pieces.