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)
(Science: physics) instantaneous center of rotation, 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. Instantaneously, Instantaneousness.