A raja is the name for a monarch — a king or a princely ruler — in historic India. While there are still rajas today, they no longer hold any real power.
Through the long history of India, there have been many rajas who ruled over small areas of the continent. Even when India was a British colony, there were princely states throughout the country that were each controlled by a raja. You can also spell the word rajah, and in either case it shows up throughout South Asia, including Sri Lanka and Malaysia. Raja comes from Hindi, with Sanskrit roots: rajan, or "king," and raj, "kingdom."