So those who have learned a second language probably have noticed that there are lexical gaps in one that aren't in the other. For example, English (and I believe at least West Germanic languages in general) has a lot of words based on the manner of motion, while many languages don't. It's impossible to translate to say French the words "slither" or "plod" without using multiple words, for example. Similarly, Korean has a huge number of onomatopoeia (and I believe words for colors) compared to most languages.
I've also read that Japanese and Dutch have a large number of words for rain (especially when you count agglutination) and Eskimo and Sami languages have a large number of synthetic words (based on polysynthesis and agglutination respectively) referring to specific forms of snow and ice. And I believe it's rare to find so many cynically precise terms for human thought and wants as in Yiddish.
This isn't quite about lexical gaps, per se. This is about a language having a sort of lexical abundance that the great majority, if not all other (at least widely spoken) languages lack. Some of these might be readily formed via compounds, (like Korean nureon (saek) is "deep yellow"), while others might be extremely hard in the context of a translation rather than a dictionary (like Yiddish oftselakhes is "the urge to do something to spite the person who just told you not to do it"), but regardless, it's a type of phrase, word, or perhaps just a morpheme that a language is nigh exceptional in the number it has.
So how would one refer to it? How would one say something like, "An example of _____ in English would be the many words for manner of motion"? It's my understanding that "lexical specialization" refers to something quite different, and while "linguistic relativity" will often include this concept, it also carries a lot more baggage with it. "Vocabulary specialization" won't get you anywhere either, except in colloquial speech.