An extreme form of skepticism, associated with Pyrrho of Elis (c.365-275 BC) and developed by his followers, notably Aenesidemus (1st century BC) and Sextus Empiricus (2nd century AD). Pyrrhonism's distinguishing feature lay in its application of scepticism to itself: not only could we not know anything, but we could not even know that we could not know anything.
Unlike the 'dogmatic' sceptics of the Academy, therefore (who did claim to know this last fact), Pyrrho advocated complete suspension of judgment and hoped to obtain a tranquil peace of mind thereby - an outlook with echoes in the philosophy of David Hume (1711-1776).
Pyrrho wrote nothing (not surprisingly), but fortunately for our historical knowledge his followers did. 'Pyrrhonism' is occasionally used loosely for skepticism in general.
A A Long and D N Sedley, The Hellenistic Philosophers (1987); translations with commentary