Which term pertains to the back part of the head?

The term that correctly pertains to the back part of the head is "occipital." In anatomical terminology, "occipital" refers specifically to the occipital bone, which is located at the posterior aspect of the skull and forms a significant part of the base of the cranium. This area is important in various medical contexts, including neurology and cranial anatomy, as it protects the brain's occipital lobes, which are responsible for visual processing.

The other terms listed do not refer to this anatomical region. "Nocuous" describes something harmful or toxic, which is unrelated to anatomy. "Nitrogenous" pertains to compounds that contain nitrogen, often discussed in biology and chemistry but not in relation to head anatomy. "Orthodontics" refers to a dental specialty focused on diagnosing and treating dental and facial irregularities, which involves the teeth and jaw rather than the back part of the head. Therefore, "occipital" is clearly the appropriate choice in this context.