A funny definition, invented by a set of elderly IT folks, as a set of IT systems that can interact without security, and a good excuse to get
architecture approved bypassing security concerns.
The definition may become so wobbly that the full landscape is a trusted boundary, the VPN is a trusted boundary, System A to System B connection is a trusted boundary, a generic combination of system A B and C is a trusted boundary.
Therefore the polymorphism of this concept has been used and misused to get
architecture approvals that bypass any poor's man
common sense.