\In`stan*ta"ne*ous\, a. [Cf. F. instantan['e].]
1. Done or occurring in an instant, or without any
   perceptible duration of time; as, the passage of
   electricity appears to be instantaneous.
         His reason saw With instantaneous view, the truth of
         things.                               --Thomson.
2. At or during a given instant; as, instantaneous
   acceleration, velocity, etc.
{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.