The word “province” and division of province has an ancient background in Iran and this background has caused its related words and concepts to be various and widespread: Satrap (Xašathrapāvan), Kust, Ostān (Province), Valayat, Mamlekat (mamleke), Balad and other toponymical terminology. Research about the types, meaning and diversity of these words is essential in order to understand fully the process of province changes in Iran. Therefore, the main question of this research is to find out what words with what kinds of backgrounds have been used to mean «Ostān (province)». The number of these words and their usages illustrates the fact that by the means of historical etymology of the word “province”, we can interpret the role of the administrative history of Iran and the major factors in utilizing the administrative terminology on a large scale. This research is a descriptive study of the etymological issues in history.