A flap pocket is a pocket on a garment whose opening has been covered by a piece of fabric. These flaps can be constructed in a number of different styles, shapes and sizes. The size and shape of the flap typically depends on what type of garment the flap is being attached to.
Typically dress shirts that have flap front pockets make use of a modified rectangle shaped flap where the lower portion of the rectangle comes to an apex (like a triangle) that is distended from the rectangle's midpoint. This is in contrast to the flaps that can be found on suit coats which are typically narrow rectangles. There are, of course, exceptions to these common practices as there is no standard definition for what type of flap pocket belongs on what type of garment. Additionally novelty shaped flaps can be found on some types of garments. Today, flap pockets are commonly used as back pockets on mens and women's designer jeans.