In phonology, segments are categorized into natural classes on the basis of their distinctive features. Each feature is a quality or characteristic of the natural class, such as voice or manner. A unique combination of features defines a phoneme.
Surface representations can be expressed as the result of rules acting on the features of the underlying representation. These rules are formulated in terms of transformations on features.
In morphology and syntax, words are often organized into lexical categories or word classes, such as "noun", "verb", "adjective", and so on. These word classes have grammatical features (also called categories or inflectional categories), which can have one of a set of potential values (also called the property, meaning, or feature of the category).
For example, consider the pronoun in English. Pronouns are a lexical category. Pronouns have the person feature, which can have a value of "first", "second", or "third". English pronouns also have the number feature, which can have a value of either "singular" or "plural". As a result, we can describe the English pronoun "they" as a pronoun with [person:3] and [number:plural].