राज, raja or राजन्, rajan

"king", "ruler", "the title of the Indian monarch"

The first mentions of the title are present in the Rigveda (for example, the Battle of the Ten Kings, Dāśarājñá yuddhá, described in the 7th mandala of the Rigveda, hymns 18, 33 and 83.4-8). The wife of the Raja, the ruler, was titled Rani (sanskrit. रानी).

The responsibility of the raja (rāja-dharma) was to maintain the true universal order of things. 

Religious functions included the worship of gods and the compliance of dharma, while the secular ones consisted of caring for the prosperity of the kingdom, maintaining justice, protecting people and their property. Raja protected the kingdom from external aggression and monitored the observance of laws in society. The king had legislative and judicial duties, he could pass punishments and resolve disputes. The raja was supposed to look after the general prosperity, and if this did not happen, it meant that he was not fulfilling his dharma well.

In modern India, Rama is considered by many people to be the ideal ruler.

After the English colonization, the Maharaja (“Great Ruler”) was often used as the title of ruler.

The word "raja"  is used in yoga and tantra: Raja Yoga (royal yoga), raja marga (royal path, an epithet of sushumna), Raja Guru (a special title in Nath Sampradaya, the head of the Kadri Math).