Geometrical Optics (or Ray Optics) : The branch of optics is treated by the method of light rays. Under ordinary situations, light travels i straight lines. This reasonable assumption has led to a ray model. The ray model is found very successful in explaining phenomena like refraction and formation of images by mirror and lenses. Since these explanation involve straight line ray at various angle, it is called geometrical optics (or ray optics).
In other words, The branch of optics which is treated by the method of light rays is called ray optic or geometrical optics.