Raja is the Hindustani term for a monarch, or princely ruler of the Kshatriya varna. The female form, the word for "queen", mainly used for a Raja's wife, is Rani , from Sanskrit rājñī.
The title has a long history in the Indian subcontinent and Southeast Asia, being attested from the Rigveda, where a rājān- is a ruler, see for example the , the "battle of ten kings".
Sanskrit rājān- is cognate to Latin rēx , Gaulish rīx etc., originally denoting tribal chiefs or heads of small 'city states'. It is ultimately derived from a PIE *h3rẽǵs, a vrddhi formation to the root *h3reǵ- "to straighten, to order, to rule". The Sanskrit n-stem is secondary in the male title, apparently adapted from the female counterpart rājñī which also has an -n- suffix in related languages, compare Old Irish rígain and Latin regina. Cognates of the word Raja in other Indo-European languages include English reign and German reich.
Wikipedia, Raja, http://en.wikipedia.org/wiki/Raja (as of Apr 1, 2010)