None

  The entropy of a random variable X is a mathematical measure of the expected amount of information provided by an observation of X. As such, entropy is always relative to an observer and his or her knowledge prior to an observation.
Source(s):
NIST SP 800-133 [Superseded]

  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 〗
Source(s):
NIST SP 800-22 Rev. 1a

  A measure of the amount of uncertainty an attacker faces to determine the value of a secret. Entropy is usually stated in bits. A value havingnbits of entropy has the same degree of uncertainty as a uniformly distributedn-bit random value.
Source(s):
NIST SP 800-63-3

  A measure of the disorder, randomness or variability in a closed system. Min-entropy is the measure used in this Recommendation.
Source(s):
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.
Source(s):
NIST SP 800-133 Rev.1 [Superseded]

  A measure of the amount of uncertainty an attacker faces to determine the value of a secret. Entropy is usually stated in bits. A value having n bits of entropy has the same degree of uncertainty as a uniformly distributed n-bit random value.
Source(s):
NIST SP 800-63-3

  A measure of the amount of uncertainty that an Attacker faces to determine the value of a secret. Entropy is usually stated in bits. See Appendix A.
Source(s):
NIST SP 800-63-2 [Superseded]