A trail or Eulerian walk is a walk in which no edge is repeated.

A stream is a trail that cannot be extended.

A stream may be formed by travelling around a polyhedron along its edges. When one encounters a corner, the journey continues by making a turn. The journey ends when it is not possible to continue without retracing an edge that has been already been traversed.

For example, consider a cube. A simple stream may be formed by starting from any edge of the cube and proceeding forward by making the following sequence of turns: right, left, left, left. These turns result in a stream that consists of the sequence of 5 edges shown in the figure below.