The Collaborative International Dictionary of English v.0.48:

Disappearance \Dis`ap*pear"ance\, n.
   The act of disappearing; cessation of appearance; removal
   from sight; vanishing. --Addison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disappearance
    n 1: the act of leaving secretly or without explanation [syn:
         disappearance, disappearing] [ant: appearance]
    2: the event of passing out of sight [ant: appearance]
    3: gradually ceasing to be visible [syn: fade,
       disappearance]
    4: ceasing to exist; "he regretted the disappearance of Greek
       from school curricula"; "what was responsible for the
       disappearance of the rainforest?"; "the disappearance of
       resistance at very low temperatures"

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "disappearance":
   AWOL, French leave, abscondence, absence, absence without leave,
   absentation, absenteeism, absenting, absquatulation, bolt,
   concealment, cut, day off, decampment, default, departure,
   desertion, disappearing act, elopement, escape, excused absence,
   exit, fleeing, flight, fugitation, furlough, hasty retreat, hegira,
   holiday, hooky, immateriality, imperceptibility, indiscernibility,
   invisibility, leave, leave of absence, leaving, nonappearance,
   nonattendance, quick exit, running away, sabbatical leave,
   scramming, secrecy, sick leave, skedaddle, skedaddling,
   the invisible, the unseen, truancy, truantism, unexcused absence,
   unperceivability, unseeableness, unsubstantiality, vacation,
   viewlessness