The SI prefix "atto" represents a factor of
10^{-18}, or in exponential notation, 1E-18.

So 1 attopascal = 10^{-18} pascals.

The definition of a pascal is as follows:

The pascal (symbol Pa) is the SI unit of pressure.It is equivalent to one newton per square metre. The unit is named after Blaise Pascal, the eminent French mathematician, physicist and philosopher.

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