Cheek refers to either side of the human face (the area below the eye and between the nose and the ear), or to either of the two hemispheres of the buttocks, the nether cheeks. The two cheeks are denoted as "left cheek" and "right cheek". It is possible to give someone a spank "on the left cheek", or to spank them "alternating cheeks".
There is also an adjective cheeky which means offensively bold, to the point of being a brat. "Cheeky" is also a popular term in spanking literature as a person is often called cheeky to imply that he or she deserves punishment to be put in line.