High Assurance Guard (HAG)

An enclave boundary protection device that controls access between a local area network that an enterprise system has a requirement to protect, and an external network that is outside the control of the enterprise system, with a high degree of assurance.
SOURCE: SP 800-32

A guard that has two basic functional capabilities: a Message Guard and a Directory Guard. The Message Guard provides filter service for message traffic traversing the Guard between adjacent security domains. The Directory Guard provides filter service for directory access and updates traversing the Guard between adjacent security domains.
SOURCE: CNSSI-4009