The attopascal is a multiple of the unit bit for Pressure. The prefix atto (symbol a) is defined in the International System of Units (SI) as a multiplier of 1.0 × 10-18 (1 quintillionth, trillionth, short scale), and therefore 1 attopascal = 1.0 × 10-18 pascals = 0.000000000000000001 pascals. The attopascal has the unit symbol aPa.
pascals
The pascal (symbol: Pa) is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus and ultimate tensile strength. It is defined as one newton per square metre.