Inflection

Inflection is a one of the main types of morphological operations by which an affix is added to a word. An inflectional affix adds a particular grammatical function to a word without changing the category of that word, or even leading to a different word. We may say that inflected forms are just variants of one and the same word.

Term proprties
The relational adjective is inflectional. An alternative spelling (confined to British English and increasingly outdated) is inflexion.

Examples
Count nouns in English can be pluralized by adding the inflectional ending -s (dog-dogs, noun-nouns). The plural forms dogs and nouns are variants of the base nouns dog and noun.

Comments
Traditionally inflection is distinguished from derivation (the second type of major morphological operation). Although it is not possible to draw a sharp boundary between both types of operation, there are at least two differences: (i) inflection is never category-changing, while derivation typically is category changing, and (ii) inflection is usually peripheral to derivation. Some linguists (e.g. Aronoff (1976), Anderson (1982), Perlmutter (1988)) assume that inflection and derivation belong to different components of the grammar. This view is not uncontroversial though, since others (e.g. Halle (1973), Kiparsky (1982)) assume that inflection and derivation are reflexes of one and the same operation, namely affixation.

Link
Utrecht Lexicon of Linguistics

Other languages
German Flexion