In physics and mathematics, peta- (symbol: P)'s a prefix in the SI (system of units) denoting 1015, or 1,000,000,000,000,000. For example: 1 petametre = 1015 metres 1 petasecond = 1015 seconds Adopted in 1975, it comes from the Greek πέντε, meaning five, because it's equal to 10005. It's based on the model of tera- (from Greek τέρας = 'immense', 'tremendous', but looking like from the Greek for "four" with a letter missing), and so peta-, coming… (More on Peta-)