morpheme

n. The smallest part of a word that carries meaning or performs a grammatical function is a morpheme. Morphemes are classified into two overarching types: free morphemes can appear alone; bound morphemes must be bound to another morpheme. Within the class of bound morphemes, derivational morphemes perform lexical operations, while inflectional morphemes have grammatical functions. For example, the derivational morpheme –ly in English is used to derive adverbs from adjectives (happy, happily), and the derivational morpheme un- negates the meaning 
of an adjective (happy, unhappy). Inflectional morphemes, responsible for marking grammatical relations among words, are a very small class in English, which includes plural –s and possessive –'s for nouns; comparative –er 
and superlative –est for adjectives; and third singular –s, past tense –ed, past participle –en, and present participle –ing for verbs. Spanish, a language with slightly more complex inflectional morphology, marks gender and number on nouns and adjectives and number, person, tense, and verb class on verbs. – EMF