Closed loop or closed-loop may refer to:
Mathematics Loop (topology), topological path whose initial point is equal to its terminal point Closed curve, the image of a continuous mapping of a circle
Technology Closed-loop controller, control law that uses knowledge of the state or output Closed-loop transfer function, mathematical function describing the net result of the effects of a feedback control loop on the input signal to the plant under control Electronic feedback loop Proportional–integral–derivative controller (PID), type of feedback controller using three terms (P, I, and D) Closed ecological system, ecosystem that does not exchange matter with the exterior Closed-loop communication, communication technique in which every received message is confirmed by the sender Ecological sanitation systems (or ecosan), approach to sanitation provision which aims to safely reuse excreta in agriculture
Other uses Circular economy, with materials recycled Closed time loop, a paradox of time travel Knot loop, in a knot