exa- [EKS uh] (Greek: from hexa-, "six"; a decimal prefix used in the international metric system for measurements).

This is a prefix that is used in the metric [decimal] system as one-quintillion [U.S.] or trillionfold [U.K.], 10¹⁸ [1 000 000 000 000 000 000]. The metric symbol for exa- is E.