What is the smallest meaningful unit of a language called?

The smallest meaningful unit of a language is called a morpheme. Morphemes can be classified into two types: free morphemes, which can stand alone as words (like "book" or "run"), and bound morphemes, which cannot stand alone and must be attached to other morphemes to convey meaning (like prefixes or suffixes such as "un-" in "undo" or "-ed" in "walked").

Understanding morphemes is crucial for language development and analysis, especially when teaching English Language Learners, as it helps in breaking down words into their constituent parts for better comprehension and vocabulary building.

The other terms relate to language but do not represent the smallest meaningful unit. A word is typically composed of one or more morphemes, a phoneme is the smallest unit of sound that distinguishes one word from another, and a lexeme refers to a set of related forms of a word that represent a single idea or concept, encompassing all grammatical variations of that root.