See studies by M. K. Naik (1972), K. K. Sharma, ed. (1980), P. Sharrad (1987), S. A. Narayan (1988), N. Nanda (1992), E. Dey (1997), A. S. Rao (1999), R. Ramachandra, ed. (2000), R. Mittapalli and P. P. Piciucco, ed. (2001), and M. Sachdey (2006).
Sanskrit word rājā cognate to Latin rēgis, the Gaulish rīx etc. (originally denoting tribal chiefs or heads of small 'city states'), ultimately a vrddhi derivation from a PIE root '' "to straighten, to order, to rule".
Rather common variants in Hindi, used for the same royal rank in (parts of) India include Rana, Rao, Raol, Rawal and Rawat. The female form, queen, mainly used for a Raja's wife, is रानि (rāni) (sometimes spelled Ranee), from Sanskrit राज्ञी (rājñī) (compare Old Irish rígain) or Thai Rajanee (Queen).
Raja, the lower title Thakore and many variations, compounds and derivations including either of these were used in and around India by most Hindu Muslim and some Buddhist and Sikh rulers, while Muslims also used Nawab or Sultan, and still is commonly used in India.
Raja is also used as a name by Hindus and Sikhs.