free particle
particle that, in some sense, is not bound by an external force, or equivalently not in a region where its potential energy varies
en-US/ˈfɹi ˈpɑɹtɪkəl/
en-GB/ˈfɹiː ˈpɑːtɪkəl/
particle that, in some sense, is not bound by an external force, or equivalently not in a region where its potential energy varies