The thickness of synthetic thread is measured in denier. Denier (den) is an old French unit of measurement that states how many grams 9,000 meters of a particular yarn weigh. This depends on the thickness of the yarn.
9,000 m of a yarn weigh 30 g = 30 den
9,000 m of a yarn weigh 70 g = 70 den
The more denier a yarn has, the stronger and more robust it is. However, this does not automatically mean the more denier the more opaque. Opaqueness depends on the composition of the polyamide, i.e. whether the filaments are thick or thin.
A thread can consist of many filaments (multifil yarn), it can be very soft and thin (small amount of denier) and yet it can be highly opaque. There are yarns that consist of only a few strong filaments. These are not so soft to the touch but they are opaque - e.g. "Miss W absolute support".
For this reason, Wolford created the term Appearance (app.) to describe the look (transparent or opaque) of a pair of tights. This term is stated on the back of a tights pack and states how the tights appear on the leg.
DENIER APPEARANCE = APPEARANCE ON THE LEG