Denier is a unit of textile measurement as per Wikipedia. It is an international system of measuring the quality of yarns and fibers -a type of yarn numbering system. It is the unit of measuring the thickness/weight of a fiber/thread (natural and synthetic).
Tex is an alternative and a new measurement unit and it tells you how many grams a 1000 m long fiber weighs (1 tex = 1 g / 1000 m).
Denier defines the mass density of the fibers that the fabric is made of and indicates the fineness of the fiber. A denier is equal to the mass in grams per 9,000 meters of thread. It is written as the weight of the strands of thread / number of strands per thread.
The denier size is an important component that has to be mentioned when selling yarns, along with number of filaments in the yarn and the twist.
A microfiber (the finest of fibers) is less than 1 denier. This means that if you weigh 9,000 meters of this fiber, it would weigh less than 1 gram.
Denier is used to measure the quality of thread of silks, and all manufactured fibers (both filament and staple). A low denier represent finer thread size and a high denier represent heavier thread size. If you have two fibers A & B and A has double the denier of B, you can be sure that A will be double the weight of B.
A fine silk fiber is 1 denier. Fibers of 2000 denier are also there – they will be very coarse and heavy.
Low denier fabrics are more pliable, soft and drapey than higher denier fabrics. They will be light weight. High denier fabrics will be more durable and stronger but heavier and rougher.They will give more warmth.
Denier is also an indication of the level of transparency of a fabric. Low denier fabrics ( with denier 5-30) are sheer, lightweight fabrics. 40+ denier knits are considered opaque. This is usually used in hosiery. A low denier stocking of 7 or higher will be extra sheer and used to make sheer stockings.
Reference : Fabric for Fashion – Laurence King