A stiff, cone-shaped collar worn around the neck, originally for medical purposes.
Their use in real life is mainly to prevent an animal from being able to reach, lick or interfere with new injuries or stitches, allowing the wound time to heal. The name comes from how animals often behave as if they're ashamed of it or see it as 'punishment' when they have to wear one, causing the medical collar to be nicknamed the "cone of shame" by owners and veterinarians.
In art, it is also sometimes used for restricting movement for BDSM/bondage purposes or to humiliate the person wearing it. Note: not to be confused with elizabethan_collar which is for the fashion collars of old, while cone_of_shame is for the medical collar commonly used by veterinarians on animals.