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.”
2.
(in English) the simple or basic form of the verb, as come, take, eat, be, used after auxiliary verbs, as in I didn't come, He must be, or this simple form preceded by a function word, as to in I want to eat.
–adjective
3.
consisting of or containing an infinitive: an infinitive construction. Abbreviation:infin.
Origin: 1425–75; late ME < LL infīnītīvus indefinite, equiv. to in-in-3+ fīnītīvus definite; see finite, -ive
in·fin·i·tive (ĭn-fĭn'ĭ-tĭv) n.
Abbr. inf. or infin. A verb form that functions as a substantive while retaining certain verbal characteristics, such as modification by adverbs, and that in English may be preceded by to, as in To go willingly is to show strength or We want him to work harder, or may also occur without to, as in She had them read the letter or We may finish today. See Usage Note at split infinitive.
[From Middle English infinitif, of an infinitive, from Old French, from Late Latin īnfīnītīvus, unlimited, indefinite, infinitive, from Latin īnfīnītus, infinite; see infinite.]
[in-fin-i-tiv] 
Grammar
ɪ