This paper proposes a definition of what it means for one system description
language to encode another one, thereby enabling an ordering of system
description languages with respect to expressive power. I compare the proposed
definition with other definitions of encoding and expressiveness found in the
literature, and illustrate it on a well-known case study: the encoding of the
synchronous in the asynchronous $\pi$-calculus.Comment: An extended abstract of this paper appeared in the proceedings of
  FoSSaCS 201