A formal Japanese word for literally "young lady", is typically used in anime when referring to wealthy, high-class female characters. The reason for that is that while she is often rich, and occasionally even an actual Blue Blood, the key point is that other people treat her like royalty, whether or not she actually is. Sometimes an Ojou can actually attain her status simply by personality alone (often in high school settings where she is probably a School Idol), by being so heavily idolized.