Infinitive (abbreviated INF) is a linguistics term for certain verb forms existing in many languages, most often used as non-finite verbs. As with many linguistic concepts, there is not a single definition applicable to all languages. The name is derived from Late Latin [modus] infinitivus, a derivative of infinitus meaning "unlimited".

In traditional descriptions of English, the infinitive is the basic dictionary form of a verb when used non-finitely, with or without the particle to. Thus to go is an infinitive, as is go in a sentence like "I must go there" (but not in "I go there", where it is a finite verb). The form without to is called the bare infinitive, and the form with to is called the full infinitive or to-infinitive.

In many other languages the infinitive is a distinct single word, often with a characteristic inflective ending, like cantar ("[to] sing") in Portuguese, morir ("[to] die") in Spanish, manger ("[to] eat") in French, portare ("[to] carry") in Latin and Italian, lieben ("[to] love") in German, читать (chitat', "[to] read") in Russian, etc. However, some languages have no infinitive forms. Many Native American languages, Arabic, Asian languages such as Japanese, and some languages in Africa and Australia do not have direct equivalents to infinitives or verbal nouns. Instead, they use finite verb forms in ordinary clauses or various special constructions.

Being a verb, an infinitive may take objects and other complements and modifiers to form a verb phrase (called an infinitive phrase). Like other non-finite verb forms (like participles, converbs, gerunds and gerundives), infinitives do not generally have an expressed subject; thus an infinitive verb phrase also constitutes a complete non-finite clause, called an infinitive (infinitival) clause. Such phrases or clauses may play a variety of roles within sentences, often being nouns (for example being the subject of a sentence or being a complement of another verb), and sometimes being adverbs or other types of modifier. Many verb forms known as infinitives differ from gerunds (verbal nouns) in that they do not inflect for case or occur in adpositional phrases. Instead, infinitives often originate in earlier inflectional forms of verbal nouns. Unlike finite verbs, infinitives are not usually inflected for tense, person, etc. either, although some degree of inflection sometimes occurs; for example Latin has distinct active and passive infinitives.