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i

Definitions:

  A counter taking integer values in the interval [1, 2r-1] that is encoded as a bit string of length r; used as an input to each invocation of a PRF in the counter mode and (optionally) in the feedback and double-pipeline iteration modes.
Sources:
NIST SP 800-108r1 [August 2022 (Includes updates as of 02-02-2024)]