What term describes a missing word in a language's vocabulary?

The term for a missing word in a language’s vocabulary is a lexical gap.

A lexical gap occurs when a language lacks a single established word for a concept that speakers may nevertheless recognize or describe with a phrase. Languages do not divide up experience in identical ways, so a concept that has a compact label in one language may require a longer explanation in another.

Lexical gaps can arise for cultural, historical, or structural reasons. A language may have no traditional need for a distinction, or a potential word may never become widely accepted. Speakers can still communicate the idea through compounds, descriptive phrases, borrowing, or newly coined terms. A lexical gap therefore does not mean the concept itself is impossible to express.

This term is often confused with a syntactic gap, which concerns an omitted element in sentence structure, or a phonological gap, which concerns unattested sound patterns. A semantic gap is a broader and less precise label, whereas lexical gap specifically concerns vocabulary.