The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Morpheme: The smallest meaningful parts of words. Morphemes are composed of phonemes, the smallest basic speech sounds in a language.
EXAMPLE: The word unworthy is composed of three morphemes: un, worth, and y. Worth is a morpheme that is also a word — and the root of the word unworthy — in itself. Trying to further break up the word worth into smaller phonemic groups would make it meaningless. Un is a morpheme that is a negator, and y is a morpheme that transforms the word unworthy into an adjective.