The probability (under the null hypothesis of randomness) that the chosen test statistic will assume values that are equal to or worse than the observed test statistic value when considering the null hypothesis. The P-valueis frequently called the “tail probability.”
Source(s):
NIST SP 800-22 Rev. 1a
The probability that the chosen test statistic will assume values that are equal to or more extreme than the observed test statistic value, assuming that the null hypothesis is true.
Source(s):
NIST SP 800-90B