Domicile
Domicile is the sign a planet rules, where it acts most freely and where classical astrology considers it strongest by sign.
Mars in Aries, Venus in Taurus and Libra, the Sun in Leo, the Moon in Cancer: in its own sign a planet is at home, and the qualities of the sign and the nature of the planet do not have to be translated for each other.
Domicile is the top tier of essential dignity. It says nothing about whether the placement is comfortable to live with; a strong Mars is still Mars.
Frequently asked
Which planets rule two signs?
In the classical scheme every planet except the Sun and the Moon rules two: one on the day side of the zodiac, one on the night side.
Is domicile better than exaltation?
It ranks higher on the dignity scale, but the two mean different things: domicile is a planet at home, exaltation is a planet honoured as a guest.