Real Dictionary

infinitive
countable noun: The infinitive of a verb is the basic form, for example 'do', 'be', ' take', and ' eat'. The infinitive is often used with 'to' in front of it.

infinitive in American English
of or connected with an infinitive
noun: a verb form found in many languages that functions as a noun or is used with auxiliary verbs, and that names the action or state without specifying the subject, as French venir “to come,” Latin esse “to be,” fuisse “to have been”

infinitive in British English
abbreviation: infinitive
noun: a form of the verb not inflected for grammatical categories such as tense and person and used without an overt subject. In English, the infinitive usually consists of the word to followed by the verb