In linguistics, the concept is called hyponymy. "Automobile" is a hyponym of "motor vehicle"; conversely, "motor vehicle" is a hypernym of "automobile".
An easy way to remember which one's which is to keep in mind that just like with other words starting with hyper- and hypo-, the former stands for "more, greater", and the latter means "less, smaller". Hypernym is for the bigger superset, while hyponym is for the smaller subcategory.
And of course a hypernym, in turn, can be a hyponym of something else, a bigger superset still. So in your example, "motor vehicle" could be a hyponym of "vehicle", which in turn could be a hyponym of "noun", which in turn could be a hyponym of "word".
So with this, you have a complete set of accurate terms, one for each side in the relationship as well as one for the relationship itself. The terminology is well-established, though the general public is more familiar with some of the other -nyms and -nymies, for other relationships, like synonyms or metonymy. Still, this site for example does have a rather well-populated dedicated tag for questions about hypernyms.