Bikini Science defines tights as a garment which covers the pelvis, legs and feet. Tights thus have but one edge, the waistline.
Be forewarned that others, including Webster's define tights also as "skintight," a condition we can certainly agree with but also "covering the body from the neck or wast down." So when you encounter the term in broader literature, such as "she danced in tights" the writer may well decree that she danced in a full body stocking.
In Bikini Science however we treat the neck down case of the tights as the unitard, its, own species, and we constrained our use of tights to indicate a waistline-down only condition; it provides a handy and precise label for this kind of garment-topology. Pantyhose are an example of specialized tights, as are the tights worn under a bathing dress. Note tights might be classified as a culotte, however because they also cover the feet they don't have, strictly speaking, leglines. That's why they are a tard and not a culotte, even though they continue and extend the "bottoms" genus to its extreme of closure.
Tights may have straps under the feet, leaving the toe and heels bare (KP88GB) but in strict terms should the garment be footless it is more correctly termed slacks, a species of culotte (and with three edges).
Hose emerge when tights fission and minus a culotte yield a pair of stockings.
Like slacks, tights can be made bulky, crafted from colorful stretch fabrics like Spandex, or made from extremely thin and sheer nylon, such as pantyhose.
Tights are a union between a culotte and stockings. Pantyhose is an example of tights, a union or panties and stockings. They were first made, in fact by sewing hose and panties together (1959).
Tights predate the emergence of swimwear but play a logical role once the bathing dress looses it bloomers (MM189510).
The bathing beauty at least two options when it comes to leg coverage--tights (which fully cover the pelvis) or the more scandal-prone stockings, which tempt the exposure of bare leg above the top of the hose. This is a well-honed tease of the era.
Tights and Layering
Tights worn over or under a leotard (a.k.a. a maillot) approximate the coverage of the unitard; this also involves layering around the pelvis. Still, the maillot tank and tights combination provides an alternative to unitard throughout the early teens. This combination also involves a layering which introduces pairings of layering, colors, and textures.
In the late teens, this layering could present a monochromatic (black) look (e.g. KY1650), or a gradation of circus color and texture (RS1650). After 1985 modern exercise wear finds resonance with tights in bright color contrasts, as well as layering with not just the maillot and culotte, but also the tanga, which fashions lines with baring the buttocks. Wearing a maillot tank tanga over top tights provides for more fashion lines to display, as well as the potential to exposure skin to the side of the hip.