guard

A computer system that (a) acts as gateway between two information systems operating under different security policies and (b) is trusted to mediate information data transfers between the two. (See: controlled interface, cross-domain solution, domain, filter. Compare: firewall.)

Usage: Frequently understood to mean that one system is operating at a higher security level than the other, and that the gateway's purpose is to prevent unauthorized disclosure of data from the higher system to the lower. However, the purpose might also be to protect the data integrity, availability, or general system integrity of one system from threats posed by connecting to the other system. The mediation may be entirely automated or may involve "reliable human review".