What is a 'Yoke' in NEC / electrician parlance?
The 'Yoke' is the structural frame of a receptacle or switch:


It is often metal, with holes for two captive mounting screws, and should almost certainly be grounded if a grounding conductor is present.

Examples:


A light switch in a single-gang junction box has a single yoke.
Receptacles (in North America) are often manufactured in a pair of two outlets on a single 'yoke', which can be installed into one single-gang junction box.