"The Visuddhimagga (Pali; English: The Path of Purification), is the 'great treatise' on Buddhist practice and Theravāda Abhidhamma written by Buddhaghosa approximately in the 5th Century in Sri Lanka. It is a manual condensing and systematizing the 5th century understanding and interpretation of the Buddhist path as maintained by the elders of the Mahavihara Monastery in Anuradhapura, Sri Lanka.
It is considered the most important Theravada text outside of the Tipitaka canon of scriptures, and is described as "the hub of a complete and coherent method of exegesis of the Tipitaka," but it has also been criticised for its non-canonical departures, and its interpretation of dhyana as concentration-meditation."
(from Wikipedia)