Closed loop or closed-loop may refer to:

• Loop (topology)
• Closed curve

• A closed feedback loop:
  • Closed-loop transfer function
  • Electronic feedback loop
  • PID controller
• Closed ecological system
• Closed-loop communication
• Ecological sanitation systems or ecosan

• Circular economy, with materials recycled
• Closed time loop, a paradox of time travel
• Knot loop, in a knot