A measure of the disorder or randomness in a closed system. The entropy of uncertainty of a random variable X with probabilities pi, …, pn is defined to be H(X)=-∑_(i=1)^n 〖p_i  log〖 p〗_i 〗
Sources:
NIST SP 800-22 Rev. 1a

  A measure of the disorder, randomness or variability in a closed system. Min-entropy is the measure used in this Recommendation.
Sources:
NIST SP 800-90A Rev. 1
NIST SP 800-90B

  A measure of the disorder, randomness, or variability in a closed system; see SP 800-90B.
Sources:
NIST SP 800-133 Rev. 2

  The amount of uncertainty that an attacker faces to determine the value of a secret. Entropy is usually stated in bits. A value with n bits of entropy has the same degree of uncertainty as a uniformly distributed n-bit random value.
Sources:
NIST SP 800-63-4 [