noun

• 1 rare Voluntary movement (of a muscle or the body).

• 2Philosophy
  The self-generated or spontaneous motion of the soul.

• 3An optical illusion in which a small stationary object in a uniform background, especially a light source in an otherwise dark field, appears to move.

  The phenomenon was first noted by A. von Humboldt in 1799, when observing stars through a telescope.