A morpheme is the smallest linguistic unit of a word that has meaning. It cannot be divided into smaller elements. For example, the word "sing" is a morpheme. It can be a syllable, affix, or root word. Derivational morphemes can be added to a word to create another word. When adding an affix, the grammatical part of speech can sometimes change. For example, the word sing is a verb, but if we add "er", the word becomes a noun, "singer".