in mechanics, vibration, or the reciprocal ascent and descent of a pendulum.

Axis of Oscillation, is a line parallel to the horizon, supposed to pass through the centre or fixed point about which the pendulum oscillates, and perpendicular to the plane in which the oscillation is made.

Centre of Oscillation, in a suspended body, is a certain point in it, such that the oscillations of the body will be made in the same time as if that point alone were suspended at that distance from the point of suspension. Or it is the point into which, if the whole weight of the body be collected, the several oscillations will be performed in the same time as before; the oscillations being made only by the force of gravity of the oscillating body.