Planets which seem to be moving in the opposite direction of their movement (when seen from Earth) in their respective orbits are termed as retrograde planets in Astrology or Vakriya (or Vakri) grahas in Jyotisha. In actual no planet moves in the opposite direction but since the Earth is also revolving around the Sun, due to which according to the concept of relative motion many times it seems that the planet is moving in opposite direction.
(i) Sun & Moon can never be retrograde.
(ii) Rahu & Ketu are always retrograde.
(iii) Planets which are not 'Vakriya' or 'Vakri' are termed as 'Maargi'.