The class of languages for which membership can be determined in polynomial time by a probabilistic Turing machine with no false acceptances and less than half false rejections. Formal Definition: For a language, S, there exists a probabilistic Turing machine, M, that accepts or rejects any string in polynomial time. If w ∉ S, M rejects w. If w ∈ S, M accepts w with a probability at least 1/2.