The answer can be found on RBI website:
NRI for this purpose is defined as a person resident outside India who is citizen of India. In terms of Regulation 2 of FEMA Notification No.13 dated May 3, 2000, Non-Resident Indian (NRI) means a person resident outside India who is a citizen of India. Person of Indian Origin (PIO) means a citizen of any country other than Bangladesh or Pakistan who had (a) at any time held Indian passport or (b) he or either of his parents or any of his grandparents was a citizen of India by virtue of the Constitution of India or the Citizenship Act, 1955 or (c) the person is a spouse of an Indian citizen or a person referred to in (a) or (b).
You can dig out RBI sites for older circulars if you want.