What is the definition of a retrograde rotation?

Answer 1

Retrograde means - sort of - backwards. To understand forward and backward, however, we need to understand the point of reference...

The planets (and asteroids and comets, etc.) revolve around the Sun. We arbitrarily called the revolution of the Earth around the Sun counter clockwise, meaning that the path is reverse that of a clock face, when you look down on the plane of the ecliptic (the path of the Earth around the Sun) from a position that is in the direction of the north pole star, or Polaris.

Using that same perspective, i.e. looking down from celestial north, we observe that most of the planets also rotate about their own axes in a counter clockwise direction. That is why the Sun appears to rise in the East and set in the West.

Six of the eight planets, Mercury, Earth, Mars, Jupiter, Saturn, and Neptune, have this "normal" counter clockwise rotation. Each planet's axis is also tilted in varying degrees to the ecliptic, but you can still consider them to be rotating counter clockwise.

Two of the planets, however, Venus and Uranus, have what we call retrograde rotation. This means that, when viewed from celestial north, they rotate clockwise, instead of counter clockwise. Their revolution (or orbit) around the Sun is still counter clockwise, but their rotation around their own axes is clockwise. That is retrograde rotation.