Edit
When a character's limbs or body is held in place by objects or devices that are made of rigid materials and locked into position such that they will not budge or sway if the victim struggles.
Key examples are pillories or stocks mounted in place, shackles bolted or screwed to a surface, (such as a wall, floor, or table) cuffs holding the subject inside a sex machine, and pipe arrangements built around the subject.
Restraints that are tethered by ropes or chains do not count, nor do tentacles. (be they organic or metallic)
The victim may have some freedom of movement or be bound by ropes or chains in addition, but at least one restraint must convey a sense of being made of solid materials and mounted firmly enough not to easily move or sway in response to the captive's struggling.
In some cases the restraints themselves may be able to move, such as a machine capable of posing the victim, but should appear the restrained subject does not have any influence upon the restraints.
See also
This tag implicates restrained (learn more).