This note provides a summary of the meaning of the term `Superselection Rule'
in Quantum Mechanics and Quantum-Field Theory. It is a slightly extended
version of a contribution to the Compendium of Quantum Physics: Concepts,
Experiments, History and Philosophy, edited by Friedel Weinert, Klaus
Hentschel, Daniel Greenberger, and Brigitte Falkenburg.Comment: 10 pages, no figures. Two easy-to-read proofs of statements made in
the text are added in the second version, with slight adaptations in notatio
