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The Collaborative International Dictionary of English v.0.48 (gcide)

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]