Pleat

A pleat (older plait) is a type of fold formed by doubling fabric back upon itself and securing it in place. It is commonly used in clothing and upholstery to gather a wide piece of fabric to a narrower circumference.

Pleats are categorized as pressed, that is, ironed or otherwise heat-set into a sharp crease, or unpressed, falling in soft rounded folds.

Pleats sewn into place are called tucks.

Fullness

A vertically hanging piece of fabric such as a skirt or a drape will often be described in terms of its "fullness." Fullness represents the thickness/ depth of the pleats in relation to the original width of the fabric: fabric sewn at "zero fullness" would actually be flat and have no pleats; fabric sewn at "100% fullness" is pleated so that it takes up exactly half as much width as it would if it were not pleated at all (i.e., 24 inches would be pleated down to 12 inches); if sewn at "150% fullness," the unpleated fabric would be two and a half times wider than the final pleated piece (i.e., an unpleated 30 inches would end up as 12 pleated inches of fabric: 12+1.50(12)=30); if fullness were to be "50%", the original fabric would be one and a half times the width of the pleated (i.e., 18 inches of width would end up as 12 pleated inches: 12+0.50(12)=18), etc.