Isocephaly is a term used in art and composition to refer to having all heads in a work of art on the same level. This was common in bas-relief and painted friezes, and is fairly common in other pieces; the human eye is drawn to the human face, allowing for a line of focus -- and also human heads tend to be on the same level in most settings anyway, so it generally does not look much out of place.
Isocephalism is relevant historically because it may distort height and size of figures; in some contexts it was not limited to humans, but also any animal that appeared in the piece (especially in friezes), resulting in giant lions or surprisingly short horses.
The term 'isocephaly' is also commonly used; the adjectival form is 'isocephalic'.