The Terminal is a new social area added to the game as part of Update 1.3. It is found at the back of the Base of Operations and initially entered via a passage next to the Rewards claim vendor at the back of the main corridor.
This area is a social space where you can find other players. It has your stash, the new Special Blueprints Vendor (which has split from the special Weapons Vendor in the Base), HVT mission Vendors and a restock crate. As well as the entrance to the Underground which is the arena for the first main expansion for the game. The passageway to the underground will take you to the TOC to participate in the missions for the dlc.