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A component of a DRBG (which consists of a DRBG mechanism and a randomness source) that outputs bitstrings that are used as entropy input by the DRBG mechanism. The randomness source can be an entropy source or an RBG.
NIST SP 800-90A Rev. 1
See Randomness Source.
NIST SP 800-90A Rev. 1 under Source of Randomness