In modern astrology the outer planets are usually defined as any planets beyond Saturn in the solar system, which usually means Uranus, Neptune, and Pluto.
The outer planets are usually classified differently from the seven classical “inner planets” because they are invisible to the naked eye, and also because they move relatively slowly through the zodiac.
These planets are usually conceptualized as signifying generational shifts due to the fact that they stay in certain zodiacal signs and configurations for very long periods of time, and thus many people are born with them in similar positions in their birth charts.
This classification of the planets was popularized in the 20th century when Uranus, Neptune, and Pluto were the outermost planetary bodies in the solar system. This was prior to the demotion of Pluto to the status of “dwarf planet” by the International Astronomical Union in 2006. Since that time astrologers have continued to use Pluto in the same way as they did prior to its demotion, and for the most part have continued to refer to it as a simply as a “planet.”
It is unclear how other recently discovered dwarf planets such as Eris will be classified by astrologers, although some have already begun investigating its astrological meaning. In the future the phrase “outer planet” may have to be redefined by astrologers in order to take into account expansions in humanity’s understanding of the solar system.