A butterfly knot is a fixed loop tied in the middle of a rope. There are a number of other knots that do the same thing, but the butterfly knot tends to work better because it doesn’t jam when strained and it’s easy to untie.
Since it’s tied in a symmetrical fashion, strain can be put on it from any direction. Even though this knot is usually tied in the middle of the rope, you can also tie it at the end of the line if you need a fixed loop that is easily untied.
The butterfly knot is a favored knot for mountain/rock climbers, used for hand or foot loops or used to hook their carabiners into. It has many uses in pioneering work.