Those information resources, mission/business processes, and/or critical programs that are of particular interest to potential or actual adversaries.
Sources:
NIST SP 800-137A under high value asset

  Information or an information system that is so critical to an organization that the loss or corruption of this information or loss of access to the system would have serious impacts on the organization’s ability to perform its mission or conduct business.
Sources:
NIST SP 800-160 Vol. 2 Rev. 1 from CISA Secure High Value Assets

  A designation of federal information or a federal information system when it relates to one or more of the following categories:
Sources:
NIST SP 800-172r3 under high value asset from OMB M-19-03

  Those assets, federal information systems, information, and data for which an unauthorized access, use, disclosure, disruption, modification, or destruction could cause a significant impact to the United States' national security interests, foreign relations, economy – or to the public confidence, civil liberties, or public health and safety of the American people.
Sources:
NISTIR 8170 under High Value Asset from OMB M-17-09