A word for "lack of a word"?

What's the word for "no word for"?

I've really been trying to find it, but I just can't seem to.

Answer

In linguistics terms, this is known as a lacuna or lexical gap.

In particular, a word that might otherwise exist, but doesn't, is known as an accidental gap, which is distinguished from systematic gaps, which are prohibited by the fundamental rules of the language in question.

From Wikipedia's article on this topic:

In linguistics an accidental gap, also known as a gap, lexical gap, lacuna, or a hole in the pattern, is a word or other form that does not exist in some language but which would be permitted by the grammatical rules of the language.

Accidental gaps differ from systematic gaps, those words or other forms which do not exist in a language due to the boundaries set by phonological, morphological, and other rules of that specific language.

Now, as that passage notes, there are several types of accidental gaps (phonological, morphological, etc), but the specific kind where a word which expresses some idea which we might otherwise expect to exist, does not in fact exist, is known as a semantic gap:

In semantics a gap may be noted when a particular meaning distinction visible elsewhere in the lexicon is absent. For example, English words describing family members generally show gender distinction. Yet the English word cousin can refer to either a male or female cousin. Similarly, while there are general terms for siblings and parents, there is no comparable gender-neutral term for an aunt or uncle.