Definition: The history of the ancient Indian subcontinent contains theories based on the Aryan Invasion hypothesis. While this theory is rejected by many archaeologists, it persists in almost every discussion of the Indus Valley civilization. In essence, the theory goes, Indo-Aryan-speaking horse-riding nomads, perhaps from around modern Iran, invaded the Indus Valley, became the dominant group, and wrote the Vedas. The theory was developed in the 19th century before the highly-developed civilizations of the Indus valley (especially Mohenjo-Daro and Harappa) were uncovered in the 20th century. When these civilizations were found, the fact that they had disappeared gave support to the theory of an invasion that wiped them out.
There is no evidence from the Vedas to support the invasion theory and Aryas, in the Sanskrit language, doesn't refer to invaders, but to nobles.
Kris Hirst, Archaeology Guide at About.com, has written a clear introduction to the Aryan Invasion Hypothesis and its connection with racism. See Who Were the Aryans? Hitler's Persistent Mythology.
