The Three Ages of Buddhism are three divisions of time following the historical Buddha's passing: the Former Day of the Law (正法 Cn: zhèngfǎ; Jp: shōbō), the first thousand years (or 500 years); the Middle Day of the Law (像法 Cn: xiàngfǎ; Jp: zōhō), the second thousand years (or 500 years); and the Latter Day of the Law (末法 Cn: mòfǎ; Jp: mappō), which is to last for 10,000 years.
The three periods are significant to Mahayana adherents, particularly those who hold the Lotus Sutra in high regard; e.g., Tiantai (Tendai) and Nichiren Buddhists, who believe that different Buddhist teachings are valid (i.e., able to lead practitioners to enlightenment) in each period due to the different capacity to accept a teaching (機根 Cn: jīgēn; Jp: kikon) of the people born in each respective period.
Further, in the Mahasamnipata Sutra, the three periods are further divided into five five-hundred year periods (五五百歳 Cn: wǔ wǔbǎi; Jp: go no gohyaku sai), the fifth and last of which was prophesied to be when the Buddhism of Sakyamuni would lose all power of salvation and a new Buddha would appear to save the people. This time period would be characterized by unrest, strife, famine, and other, natural disasters.
The three periods and the five five-hundred year periods are described in the Sutra of the Great Assembly (大集経 Cn: dàjí; Jp: Daishutu-kyō, Daijuku-kyō, Daijikkyō, or Daishukkyō). Descriptions of the three periods also appear in other sutras, some of which ascribe different lengths of time to them (although all agree that Mappō will last for “10,000” years, though rather than a concrete figure, this merely signifies a long period of time).