Function words are words that have little lexical meaning or have ambiguous meaning, but instead serve to express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker. Words which are not function words are called content words or lexical words: these include nouns, verbs, adjectives, and most adverbs, though some adverbs are function words (e.g. then, why). Dictionaries define the specific meanings of content words, but can only describe the general usages of function words. By contrast, grammars describe the use of function words in detail, but have little interest in lexical words.

Function words may be prepositions, pronouns, auxiliary verbs, conjunctions, grammatical articles or particles, all of which belong to the group of closed class words. Interjections are sometimes considered function words but they belong to the group of open class words. Function words may or may not be inflected or may have affixes.

They belong to the closed class of words in grammar in that it is very uncommon to have new function words created in the course of speech, whereas in the open class word, that is nouns, verbs, adjectives, or adverbs, new words may be formed readily (such as slang words, technical terms, adoptions and adaptations of foreign words). See neologism.

Each function word gives some grammatical information on other words in a sentence or clause, and cannot be isolated from other words, or it may indicate the speaker's mental position as to what is being said.

Here follows a list of the type of words included in function words:

• articles - the and a - only a, the indefinite article is inflected with an affix n before a noun starting with a vowel in English. In highly inflected languages, the articles take on the case of the declension of the following noun.
• pronouns - inflected in English, as he- him, she - her, etc
• prepositions - uninflected in English.
• conjunctions - uninflected
• auxiliary verbs - forming part of the conjugation (pattern of the tenses of main verbs) are always inflected
• interjections - sometimes called "filled pauses", are uninflected
• particles - convey the attitude of the speaker and are uninflected, as if, then, well, however, thus, etc.
• expletives - set up sentences, and other functions, It is, There are, etc.