The most commonly used recent definitions are:
Hominid – the group consisting of all modern and extinct Great Apes (that is, modern humans, chimpanzees, gorillas and orang-utans plus all their immediate ancestors).
Hominin – the group consisting of modern humans, extinct human species and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus and Ardipithecus).
Current use of the term ‘hominid’ can be confusing because the definition of this word has changed over time.
The term ‘hominid’ used to have the same meaning that ‘hominin’ now has. It was therefore a very useful term to designate the line leading to modern humans and was used when referring to various members of our human evolutionary tree.
‘Hominid’ has now been assigned a broader meaning and now refers to all Great Apes and their ancestors. This new terminology is being used in many scientific journals already, and it is only a matter of time (but possibly many years) before everyone catches up to using the new term.
The problem for students and teachers is that a lot of texts still use the old system and many internet sites also haven't caught up, even those of many reputable scientific establishments. So students/teachers will need to be aware that 'hominids' can mean two different things depending on how up-to-date a reference is with regard to incorporating these taxonomy/classification changes.
Why have these changes occurred?
‘Hominid’ and ‘hominin’ are derived from names used in the scientific classification of apes (including humans). By international convention, certain word endings are used for specific taxons or levels within this classification. For example, ‘family’ names always end in ‘-idae’ (eg Hominidae), ‘subfamily’ names end in ‘inae’ (eg Homininae) and ‘tribe’ (1) names end in ‘ini’ (eg Hominini). These formal names are then abbreviated to give the common names hominid, hominine and hominin respectively.
The name changes that have occurred have arisen due to changes in the way humans, chimpanzees, gorillas and orang-utans are classified. For example, it was once thought that apes should be divided into three different groups (families). In this old scheme, humans were seen to be so different to other apes that we should be placed into our own distinct family, the Hominidae or hominids.
Over time, biological classifications change due to improved techniques and better knowledge about the biology and the evolutionary relationships of different living things. Now, with their better knowledge, scientists have revised their classifications to develop more up-to-date evolutionary trees. In this scheme, only two families are recognised with all the Great Apes (including humans) placed into the same family, the Hominidae or hominids. The next branching of this evolutionary tree divides the orang-utans into one subfamily and all the remaining Great Apes into another subfamily. Then at the tribe level, gorillas, chimpanzees and humans separate onto different branches of the evolutionary tree with humans in the Hominini or hominin branch. As a result of this classification change, modern humans and all our extinct ancestors on our own branch of the evolutionary tree are now known as hominins rather than as hominids as they were formerly known in old classifications.
(1) Note: The word ‘tribe’ as used here and in biological classification has a particular meaning. It refers to a taxonomic group that occurs in the classification hierarchy between subfamily and genus. If using this terminology with students, ensure that they understand this word’s meaning in a taxonomic context rather than in other contexts where it may mean ‘a social division of our own species, Homo sapiens’.