The description of an enterprise’s entire set of information systems: how they are configured, how they are integrated, how they interface to the external environment at the enterprise’s boundary, how they are operated to support the enterprise mission, and how they contribute to the enterprise’s overall security posture.
Sources:
NIST SP 800-128 under Enterprise Architecture from CNSSI 4009
NIST SP 800-30 Rev. 1 under Enterprise Architecture from CNSSI 4009
NIST SP 800-39 under Enterprise Architecture from CNSSI 4009

  A strategic information asset base, which defines the mission; the information necessary to perform the mission; the technologies necessary to perform the mission; and the transitional processes for implementing new technologies in response to changing mission needs; and includes a baseline architecture; a target architecture; and a sequencing plan.
Sources:
NIST SP 800-128 under enterprise architecture from 44 U.S.C., Sec. 3601
NIST SP 800-37 Rev. 2 under enterprise architecture from 44 U.S.C., Sec. 3601
NIST SP 800-53 Rev. 5 under enterprise architecture from OMB Circular A-130 (2016)

  A strategic information asset base that defines the mission, the information necessary to perform the mission, the technologies necessary for performing the mission, and the transitional process for implementing new technologies in response to changing mission needs. The EA includes a baseline architecture, target architecture, and sequencing plan.
Sources:
CNSSI 4009-2015 from CNSSP 24