NIST SP 800-22r1
Entropy Source - 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.
NIST SP 800-90
Entropy Source - A source of unpredictable data. There is no assumption that the unpredictable data has a uniform distribution. The entropy source includes a noise source, such as thermal noise or hard drive seek times; a digitization process; an assessment process; an optional conditioning process and health tests. Contrast with the Source of Entropy Input.