instantaneous axis of rotation

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