Also referred to as an athletic cup, sports cup, compression cup, box, pelvic protector, L(aura) guard, Abdo(minal) guard, penis guard, or ball guard

Usually inserted into a jockstrap, bike_shorts/compression shorts, or any other underwear with a cup pocket,
it is a hard plastic shell, often in the shape of a half-pear, designed to be worn over the user's genitals to protect their crotch from impacts during sports or any other athletic event, such as:

• american_football
• hockey
• lacrosse
• baseball_(sport)
• cricket_(sport)
• Martial_arts
• boxing
• paintball

Certain protective cups may have attached straps and/or padding to be worn over or in favor of the user's underwear, resembling a jockstrap, such as in:
post #185650 post #659534 (On his tail)
But in these cases, can be differentiated by it keeping the shape, and having venting perforations.
In these cases, it's probably best to tag both protective_cup and jockstrap, as it is a sort of combination device

Note: This is similar to a codpiece, but a codpiece is worn on top of clothing, and did not necessarily serve as a protective item, though armored codpieces do exist (though they were rarely, if ever, actually used in battle), the primary purpose of a codpiece is for aesthetics/accentuating ones bulge. A protective_cup on the other hand, is worn inside the user's clothing and the primary purpose is protecting the user's genitals from impacts.
As a side note on armored codpieces: codpiece is also tagged on "futuristic" armor that has a specific plate for the user's crotch, as it is worn on the outside.

post #947769 post #355032
shows off underwear with a couple button_(fastener)s on it, meant to hold the cup in during use.