In linguistics, a grammatical word belongs to one of the "closed" parts of speech such as pronouns, numerals, and prepositions, which do not readily admit new members. We have only to consider the failure of proposals for a gender neutral pronoun for English, to see how difficult it is to create a new pronoun that is actually adopted by speakers of a language.

This contrasts with lexical words, which belong to the "open" parts of speech, such as nouns, verbs, and adjectives. People coin new lexical words all the time; a popular song or presidential speech is all it takes to add a new lexical word to the language. That is, these parts of speech are "open" to new additions.

Lexical and grammatical words often behave differently. Often grammatical words to not make full use of all the sounds in a language. For example, in some of the Khoisan languages, most lexical words begin with clicks, but very few grammatical words do. In Mandarin Chinese, many grammatical words lack tone, but every lexical word has a tone.