Final answer:
Hominin refers to species that are in direct lineage to humans, whereas hominid refers to all modern and extinct great apes and their ancestors. Bipedal locomotion is a defining characteristic of hominins.
Step-by-step explanation:
Hominin:
The term hominin refers to all species considered to be in direct lineage to humans, which include the genera Homo, Australopithecus, Paranthropus, and Ardipithecus. Hominins differ from other primates in their anatomical and behavioral characteristics, with the most significant being bipedal locomotion. This term is used to designate species that are more closely related to humans than to chimpanzees.
Hominid:
The term hominid refers to all modern and extinct great apes, which include humans, gorillas, chimpanzees, and orangutans and their ancestors. Hominids share a common ancestor and have similar biological traits such as a larger brain size, bipedal locomotion, and an opposable thumb.