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]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "envelopment":
   bandage, bandaging, beleaguerment, besetment, besiegement, binder,
   binding, blanketing, blockade, blockading, blocking, blotting out,
   cementwork, cincture, circling, circumambience, circumambiency,
   circumcincture, circumflexion, circumjacence, circumposition,
   circumscription, cloaking, clouding, coating, confinement,
   containment, cordoning, coverage, covering, curtaining,
   dust jacket, eclipse, eclipsing, embracement, encincture,
   encirclement, enclosure, encompassment, enfoldment, envelope,
   environment, enwrapment, enwrapping, gift wrapping, girding,
   girdling, hiding, immurement, imprisonment, incarceration,
   inclusion, incrustation, investment, involvement, jacket,
   laying on, mantling, masking, obduction, obscuring, occultation,
   overlaying, overspreading, pargeting, pincer movement, plasterwork,
   quarantine, screening, sheathing, shielding, shrouding, siege,
   stuccowork, superimposition, superposition, surrounding,
   upholstering, upholstery, veiling, vertical envelopment, wrap,
   wrapper, wrapping