This is when the motion of a planet on the celestial sphere momentarily ceases to move with respect to the fixed stars. This happens twice during the retrograde loop for superior planets and occurs at greatest elongation for the inferior planets.

In mathematics, particularly in calculus, a stationary point is an input to a function where the gradient is zero. For the graph of a one-dimensional function, this corresponds to a point on the graph where the tangent is parallel to the x-axis. For the graph of a two-dimensional function, this corresponds to a point on the graph where the tangent parallel to the XY plane.