closed loop
closed-loop system

Consider a closed loop of string, left to move through space without external forces.

They are connected to each other in a closed loop.

An example of this would be holding one's shoulder to create a closed loop which the other arm goes through.

In other words, curl is always present when a field line forms a closed loop.

It had been bent so that the open end was at a right angle to the closed loop.

The second system is a closed loop and we can't touch it.

The second is never to leave on the floor a closed loop of wire or string.

Water is then pumped into the system, which forms a closed loop.

According to this definition, a knot only makes sense in a closed loop.

If the closed loop were long enough it could even be used for power transmission.