Source : Webster's Revised Unabridged Dictionary (1913)

Infinitive \In*fin"i*tive\, n. [L. infinitivus: cf. F.
   infinitif. See {Infinite}.]
   Unlimited; not bounded or restricted; undefined.

   {Infinitive mood} (Gram.), that form of the verb which merely
      names the action, and performs the office of a verbal
      noun. Some grammarians make two forms in English: ({a})
      The simple form, as, speak, go, hear, before which to is
      commonly placed, as, to speak; to go; to hear. ({b}) The
      form of the imperfect participle, called the infinitive in
      -ing; as, going is as easy as standing.

   Note: With the auxiliary verbs may, can, must, might, could,
         would, and should, the simple infinitive is expressed
         without to; as, you may speak; they must hear, etc. The
         infinitive usually omits to with the verbs let, dare,
         do, bid, make, see, hear, need, etc.; as, let me go;
         you dare not tell; make him work; hear him talk, etc.

   Note: In Anglo-Saxon, the simple infinitive was not preceded
         by to (the sign of modern simple infinitive), but it
         had a dative form (sometimes called the gerundial
         infinitive) which was preceded by to, and was chiefly
         employed in expressing purpose. See {Gerund}, 2.

   Note: The gerundial ending (-anne) not only took the same
         form as the simple infinitive (-an), but it was
         confounded with the present participle in -ende, or
         -inde (later -inge).

Infinitive \In*fin"i*tive\, n. (Gram.)
   An infinitive form of the verb; a verb in the infinitive
   mood; the infinitive mood.

Infinitive \In*fin"i*tive\, adv. (Gram.)
   In the manner of an infinitive mood.

Source : WordNet®

infinitive
     adj 1: formed with the infinitive; "an infinitive phrase"
     2: not having inflections to indicate tense
     n : the uninflected form of the verb