Source : Webster's Revised Unabridged Dictionary (1913)

En- \En-\
   1. [F. en-, L. in.] A prefix signifying in or into, used in
      many English words, chiefly those borrowed from the
      French. Some English words are written indifferently with
      en-or in-. For ease of pronunciation it is commonly
      changed to em-before p, b, and m, as in employ, embody,
      emmew. It is sometimes used to give a causal force, as in
      enable, enfeeble, to cause to be, or to make, able, or
      feeble; and sometimes merely gives an intensive force, as
      in enchasten. See {In-}.

   2. A prefix from Gr. ? in, meaning in; as, encephalon,
      entomology. See {In-}.