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. Word can mean one of several things: A linguistic word—a unit of language that symbolizes or communicates a meaning, consisting of one or more morphemes. ... A lexicon is a list of words together with additional word-specific information, i. ... Meaning, studied in philosophy and linguistics, as well as being central to the fields of literary theory and critical theory, the philosophical field of epistemology, and some branches of psychoanalysis, is a difficult concept to pin down. ... This article is about grammar from a linguistic perspective. ... In linguistics, the sentence is a unit of language, characterised in most languages by the presence of a finite verb. ... A noun, or noun substantive, is a word or phrase that refers to a person, place, thing, event, substance or quality. ... A verb is a part of speech that usually denotes action (bring, read), occurrence (to decompose (itself), to glitter), or a state of being (exist, live, soak, stand). Depending on the language, a verb may vary in form according to many factors, possibly including its tense, aspect, mood and voice. ... An adjective is a part of speech which modifies a noun, usually making its meaning more specific. ... An adverb is a part of speech that normally serves to modify verbs, adjectives, other adverbs, clauses, and sentences. ... For other uses of dictionary, see dictionary (disambiguation). ... This article is about grammar from a linguistic perspective. ...

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. In grammar, a preposition is a word that establishes a relationship between an object (usually a noun phrase) and some other part of the sentence, often expressing a location in place or time. ... In linguistics and grammar, a pronoun is a word that usually takes the place of a noun or noun phrase that was previously mentioned (such as she, it) or that refers to something or someone (I, me, you). Pronouns are often one of the basic parts of speech of the... In linguistics, an auxiliary verb is a verb whose function it is to give further semantic information about the main verb which follows it. ... A conjunction is a part of speech that connects two words, phrases, or clauses together. ... An article is a word that is put next to a noun to indicate the type of reference being made to the noun. ... In linguistics, the term particle is often employed as a useful catch-all lacking a strict definition. ... A closed word class, in linguistics, is a word class to which no new items can normally be added, and that usually contains a relatively small number of items. ... An interjection, sometimes called a filled pause, is a part of speech that usually has no grammatical connection to the rest of the sentence and simply expresses emotion on the part of the speaker, although most interjections have clear definitions. ... An open word class, in linguistics, is a word class that accepts the addition of new items, through such processes as compounding, derivation, coining, borrowing, etc. ... This article is about inflection in linguistics. ... An affix is a morpheme that is attached to a base morpheme to form a word. ...

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. This article is about grammar from a linguistic perspective. ... An open word class, in linguistics, is a word class that accepts the addition of new items, through such processes as compounding, derivation, coining, borrowing, etc. ... Slang is the non-standard use of words in a language of a particular social group, and sometimes the creation of new words or importation of words from another language. ... In linguistics, a neologism is a recently coined word, or the act of inventing a word or phrase. ...

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.