A Power Inlet.
A Power Inlet is a structure that allows the player to provide power to a section of a derelict.
They can be activated with either a Generator upgrade, or by using the Remote Power ship upgrade.
Each power inlet can only power a set number of rooms; if only part of a derelict is powered, then there is more than one inlet on the ship.
The Reroute Power ship upgrade allows the player to see how many rooms an inlet can power, and allows them to change which rooms are powered by an inlet.
Power inlets will never generate in the same room as a defense turret.