A physical source of information whose output either appears to be random in itself or by applying some filtering/distillation process. This output is used as input to either a RNG or PRNG.
Source(s):
NIST SP 800-22 Rev. 1a
A combination of a noise source (e.g., thermal noise or hard drive seek times), health tests, and an optional conditioning component. The entropy source produces random bitstrings to be used by an RBG.
Source(s):
NIST SP 800-90A Rev. 1
The combination of a noise source, health tests, and an optional conditioning component that produce random bitstrings to be used by an RBG.
Source(s):
NIST SP 800-90B
under Entropy source