A major application, general support system, high impact program, physical plant, mission critical system, personnel, equipment, or a logically related group of systems.
Sources:
CNSSI 4009-2015

  An item of value to stakeholders. An asset may be tangible (e.g., a physical item such as hardware, firmware, computing platform, network device, or other technology component) or intangible (e.g., humans, data, information, software, capability, function, service, trademark, copyright, patent, intellectual property, image, or reputation). The value of an asset is determined by stakeholders in consideration of loss concerns across the entire system life cycle. Such concerns include but are not limited to business or mission concerns.
Sources:
NIST SP 800-160 Vol. 2 Rev. 1

  Anything that has value to a person or organization.
Sources:
NIST SP 800-160v1r1 from ISO/IEC/IEEE 24765:2017

  Anything that has value to an organization, including, but not limited to, another organization, person, computing device, information technology (IT) system, IT network, IT circuit, software (both an installed instance and a physical instance), virtual computing platform (common in cloud and virtualized computing), and related hardware (e.g., locks, cabinets, keyboards).
Sources:
NISTIR 7693 under Asset
NISTIR 7694 under Asset

  Resources of value that an organization possesses or employs.
Sources:
NISTIR 8011 Vol. 1 under Asset

  Anything that can be transferred.
Sources:
NISTIR 8202 under Assets

  The data, personnel, devices, systems, and facilities that enable the organization to achieve business purposes.
Sources:
NISTIR 8286 under Assets from NIST Cybersecurity Framework Version 1.1