A planet in its own sign
A planet in its own sign is a planet in a sign it rules. The term is about the sign, not the house, and it describes the comfort the planet has there in the traditional reading.
What it is
The own signs are Sun in Simha, Moon in Karka, Mars in Mesha and Vrishchika, Mercury in Mithuna and Kanya, Jupiter in Dhanu and Meena, Venus in Vrishabha and Tula, and Saturn in Makara and Kumbha.
How it is worked out or read
A planet in its own sign is considered strong in the classical rules, and it may still be placed in a difficult house. Both the sign and the house are read together.
Where it appears on this site
The own signs are on the own sign page, and the planets are in the birth chart.
A planet in its own sign in a difficult house
A planet in its own sign may still be placed in a house that tests it, such as the sixth or the twelfth. The reader then describes the planet as comfortable in its sign but tested by its setting, and the reading is more nuanced than a simple label of strength.
Own sign and own house
The phrase own house is sometimes used loosely, and it should be kept distinct from own sign. A planet's own sign is a sign it rules, while a house is a position counted from the Lagna. A planet can be in its own sign while sitting in any house, and the two are read separately before being combined.
The distinction matters for reading, because the dignity of a planet in its own sign comes from the sign's lord, while the meaning of its house comes from the position in the chart.
The own signs of each graha
The Sun and Moon each have one own sign: Simha for the Sun and Karka for the Moon. Mars has Mesha and Vrishchika, Mercury has Mithuna and Kanya, Jupiter has Dhanu and Meena, Venus has Vrishabha and Tula, and Saturn has Makara and Kumbha. Rahu and Ketu are not given own signs in the standard table, and some schools assign them signs of their own.
A reader who knows these pairs can recognise a planet in its own sign at a glance, and then proceed to consider the house in which it sits.
What own sign suggests
In the classical view a planet in its own sign acts with ease and comfort. It is considered strong for its natural expression, and its results are read as coming more readily. This is a general dignity, and it is weighed with the house, the aspects and the lords.
Keeping the dignity in view
A planet in its own sign is not guaranteed a favourable placement in every house. The reader should describe the planet as comfortable in its sign and then describe how its house and its aspects shape that comfort. A balanced reading gives both the dignity and the setting.