Final answer:
A planar (flat) posterior headrest is a flat headrest positioned at the back of the head to provide support and minimize pressure, corresponding to option 1.
Step-by-step explanation:
A planar (flat) posterior headrest is a type of headrest designed to support the back of the head and is characterized by its flat surface. This definition corresponds to option 1) A headrest that is flat and positioned at the back of the head. The term 'posterior' indeed refers to the back or direction toward the back of the body, and in this context, it means that the headrest is situated to support the rear part of the head.
Conversely, a 'planar' surface is flat, so a planar posterior headrest comprises a flat surface that comes in contact with the back of the head, potentially to minimize the pressure exerted in that area, particularly when in a prone (face down) position. Such headrests may be important in various settings, including medical scenarios pertaining to regional anatomy studies or patient care, providing support and comfort. These headrests can be contrasted with those that are adjustable, curved, or removable, as noted in the other options provided.