All components of an information system to be authorized for operation by an authorizing official and excludes separately authorized systems, to which the information system is connected.
Sources:
CNSSI 4009-2015
NIST SP 800-137 under Authorization Boundary
NIST SP 800-30 Rev. 1 under Authorization Boundary from CNSSI 4009
NIST SP 800-39 under Authorization Boundary

  A discrete identifiable IT asset that represents a building block of an information system.
Sources:
NIST SP 800-128 under Information System Component

  All components of an information system to be authorized for operation by an authorizing official. This excludes separately authorized systems to which the information system is connected.
Sources:
NIST SP 800-161r1 from NIST SP 800-53 Rev. 5
NIST SP 800-37 Rev. 2 from OMB Circular A-130 (2016)
NIST SP 800-53 Rev. 5 from OMB Circular A-130 (2016)
NIST SP 800-53A Rev. 5 from OMB Circular A-130 (2016)

  See Authorization Boundary.
Sources:
CNSSI 4009-2015 under information system boundary
NIST SP 800-137 under Information System Boundary
NIST SP 800-30 Rev. 1 under Information System Boundary
NIST SP 800-37 Rev. 2 under information system boundary
NIST SP 800-37 Rev. 2 under system boundary
NIST SP 800-39 under Information System Boundary