A planet whose orbit lies outside the asteroid belt, i.e. Jupiter, Saturn, Uranus, or Neptune.
The outer planets are those planets in the Solar System beyond the asteroid belt, and hence refers to the gas giants and ice giants, which are in order of their distance from the Sun.
After Mars, there is an asteroid belt that separates the inner planets from the outer ones. The outer planets are called the Jovian planets, meaning huge gas giant. These planets are,
1. Jupiter
2. Saturn
3. Uranus
4. Neptune.
Read: What are Inner planets in our solar system?