The Collaborative International Dictionary of English v.0.48:

Envelopment \En*vel"op*ment\, n. [Cf. F. enveloppement.]
   1. The act of enveloping or wrapping; an inclosing or
      covering on all sides.
      [1913 Webster]

   2. That which envelops or surrounds; an envelop.
      [1913 Webster]

WordNet (r) 3.0 (2006):

envelopment
    n 1: the act of enclosing something inside something else [syn:
         enclosure, enclosing, envelopment, inclosure]