In grammar, infinitive is the name for certain verb forms that exist in many languages. In the usual (traditional) description of English, the infinitive of a verb is its basic form with or without the particle to: therefore, do and to do, be and to be, and so on are infinitives. As with many linguistic concepts, there is not a single definition of infinitive that applies to all languages. Many Native American languages and some languages in Africa and Aboriginal Australia simply do not have infinitives or verbal nouns. In their place they use finite verb forms used in ordinary clauses or special constructions.

In languages that have infinitives, they generally have most of the following properties:

• In most uses, infinitives are non-finite verbs.
• They function as other lexical categories — usually nouns — within the clauses that contain them, for example by serving as the subject of another verb.
• They do not represent any of the verb's arguments (as employer and employee do).
• They are not inflected to agree with any subject
• They cannot serve as the only verb of a declarative sentence.
• They do not have tense, aspect, moods, and/or voice, or they are limited in the range of tenses, aspects, moods, and/or voices that they can use. (In languages where infinitives do not have moods at all, they are usually treated as being their own non-finite mood.)
• They are used with auxiliary verbs.

However, it bears repeating that none of the above is a defining quality of the infinitive; infinitives do not have all these properties in every language, as it is shown below, and other verb forms may have one or more of them. For example, English gerunds and participles have most of these properties as well.