The Collaborative International Dictionary of English v.0.48:

Instantaneous \In`stan*ta"ne*ous\, a. [Cf. F. instantan['e].]
   [1913 Webster]
   1. Done or occurring in an instant, or without any
      perceptible duration of time; as, the passage of
      electricity appears to be instantaneous.
      [1913 Webster]

            His reason saw
            With instantaneous view, the truth of things.
                                                  --Thomson.
      [1913 Webster]

   2. At or during a given instant; as, instantaneous
      acceleration, velocity, etc.
      [1913 Webster]

   Instantaneous center of rotation (Kinematics), in a plane
      or in a plane figure which has motions both of translation
      and of rotation in the plane, is the point which for the
      instant is at rest.

   Instantaneous axis of rotation (Kinematics), in a body
      which has motions both of translation and rotation, is a
      line, which is supposed to be rigidly united with the
      body, and which for the instant is at rest. The motion of
      the body is for the instant simply that of rotation about
      the instantaneous axis. -- In`stan*ta"ne*ous*ly, adv. --
      In`stan*ta"ne*ous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

instantaneous
    adj 1: occurring with no delay; "relief was instantaneous";
           "instant gratification" [syn: instantaneous,
           instant(a)]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "instantaneous":
   alert, apt, brief, compact, compendious, concise, curt, curtal,
   curtate, decisive, decurtate, expeditious, fast, hair-trigger,
   immediate, instant, lightning-like, little, low, momentaneous,
   momentary, presto, prompt, punctual, quick, quick as lightning,
   quick as thought, rapid, ready, short, short and sweet, speedy,
   spontaneous, succinct, summary, swift, synoptic, transient,
   transitory