three periods of the teachings 

A way of schematizing Buddhist history that arose in China during the period of disunity following the fall of the Han dynasty in the early 3rd century ce. According to this teaching, the history of Buddhism divides into three periods. First is the Age of the True Dharma, which lasts 500 or 1,000 years, depending on the source. This is the age right after the death of a Buddha, when the teaching is vigorous, people are capable of comprehending it and putting it into practice, and many attain enlightenment (bodhi) under their own power. Second is the Age of the Counterfeit Dharma, in which the true teachings become obscured and only a semblance of it exists. At this time, which again lasts for 500 or 1,000 years depending on the source, only a few people of great intelligence are able to grasp the doctrine correctly and attain enlightenment. Third was the Age of the Final Dharma (Chinese, mo-fa; Jap., mappō), in which people's capacities were feeble and the teachings were lost. Many Buddhists in east Asia during the medieval period felt that this age was either imminent or had already arrived. The resulting loss of faith in traditional practices and teachings as means of gaining liberation led to the popularization of new modes of practice that relied more on the power of an already-enlightened Buddha (in the case of Pure Land Buddhism) or a text (in Nichiren Buddhism) for liberation.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)