circuit is one that is not connected to a main circuit board
in an electronic system. For example, a computer graphics card
is one of the components essential to the proper operation of the computer, but it is not directly attached to the motherboard
like a RAM
module or CPU
The term may also apply to other, non-electronic systems such as closed-circuit rebreathers. Divers may refer to gas supplies that are detached from the main system as "offboard".