A term (Lat. perperum; lit. "highly refined") initially applied to the gold coin of standard weight, but only 20 1/2 carats fine, introduced by Alexius I in 1092. By extension, a money of account based on this coin. After the gold hyperpyron, by then much debased, ceased to be struck in the mid-fourteenth century, the name was transferred to a large silver coin that replaced it, though the latter had only half the former 's value.
Source: Source: Grierson, Philip. Byzantine Coins. (London, 1999).