A set of unambiguously specified transformations that provide a (cryptographic) service when properly implemented and maintained. A scheme is a higher-level construct than a primitive and a lower-level construct than a protocol.
Source(s):
NIST SP 800-56B Rev. 2

  A set of unambiguously specified transformations that provide a (cryptographic) service (e.g., key establishment) when properly implemented and maintained. A scheme is a higher-level construct than a primitive and a lower-level construct than a protocol.
Source(s):
NIST SP 800-175B Rev. 1

  Set of rules and procedures that describes the objects of conformity assessment, identifies the specified requirements and provides the methodology for performing conformity assessment.
Source(s):
Cybersecurity Labeling for Consumer IoT Products