Ultra sheer refers to very light deniers of stocking

Stocking

A stocking, , is a close-fitting, variously elastic garment covering the foot and lower part of the leg. Stockings vary in color, design and transparency...

s or pantyhose

Pantyhose

Pantyhose are sheer, close-fitting legwear, covering the wearer's body from the waist to the feet. Mostly considered to be a woman's and girl's garment, pantyhose appeared in the 1960s, and they provided a convenient alternative to stockings...

, usually 10 or less. The denier of a stocking refers to the thickness of the nylon

Nylon

Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

 yarn

Yarn

Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitting, weaving, embroidery and ropemaking. Thread is a type of yarn intended for sewing by hand or machine. Modern manufactured sewing threads may be finished with wax or...

 used in the fabric. The greater the denier, the more durable the material and less prone to tearing (or "getting a run"). Ultra sheer stockings have a very light transparency

Transparency and translucency

In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...

 and a high sheen

Sheen

Sheen could mean:Places:*In south west London, England:** Sheen or West Sheen, an alternative name for Richmond, London** East Sheen** North Sheen** Sheen Priory* Sheen, Staffordshire, United Kingdom...

.

Sources

• Glossary of hosiery terms