The abbreviation for 'Before Present', used especially in radiocarbon dating. The fixed reference date for 'Before Present' has been established as 1950 AD. Thus, 4250 BP would mean 4250 years prior to 1950, or 2300 BC. The year 1950 was the latest that the atmosphere was sufficiently uncontaminated to act as a standard for radiocarbon dating. The lower case 'bp' represents uncalibrated radiocarbon years; the capitals BP denote a calibrated radiocarbon date, or a date derived from some other dating method, such as potassium-argon, that does not need calibration.