The Collaborative International Dictionary of English v.0.48:

Decease \De*cease"\, n. [OE. deses, deces, F. d['e]c[`e]s, fr.
   L. decessus departure, death, fr. decedere to depart, die;
   de- + cedere to withdraw. See Cease, Cede.]
   Departure, especially departure from this life; death.
   [1913 Webster]

         His decease, which he should accomplish at Jerusalem.
                                                  --Luke ix. 31.
   [1913 Webster]

         And I, the whilst you mourn for his decease,
         Will with my mourning plaints your plaint increase.
                                                  --Spenser.

   Syn: Death; departure; dissolution; demise; release. See
        Death.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decease \De*cease"\, v. i. [imp. & p. p. Deceased; p. pr. &
   vb. n. Deceasing.]
   To depart from this life; to die; to pass away.
   [1913 Webster]

         She's dead, deceased, she's dead.        --Shak.
   [1913 Webster]

         When our summers have deceased.          --Tennyson.
   [1913 Webster]

         Inasmuch as he carries the malignity and the lie with
         him, he so far deceases from nature.     --Emerson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decease
    n 1: the event of dying or departure from life; "her death came
         as a terrible shock"; "upon your decease the capital will
         pass to your grandchildren" [syn: death, decease,
         expiry] [ant: birth, nascence, nascency,
         nativity]
    v 1: pass from physical life and lose all bodily attributes and
         functions necessary to sustain life; "She died from
         cancer"; "The children perished in the fire"; "The patient
         went peacefully"; "The old guy kicked the bucket at the age
         of 102" [syn: die, decease, perish, go, exit,
         pass away, expire, pass, kick the bucket, cash in
         one's chips, buy the farm, conk, give-up the ghost,
         drop dead, pop off, choke, croak, snuff it] [ant:
         be born]

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "decease":
   Z, annihilation, apodosis, bane, be lost, biological death,
   cash in, catastrophe, cease to be, cease to live, ceasing,
   cessation, cessation of life, clinical death, coda, conclusion,
   consummation, crack of doom, crossing the bar, culmination,
   curtain, curtains, death, death knell, debt of nature, demise,
   denouement, depart, depart this life, departure, destination,
   destiny, die, dissolution, doom, drop, dying, ebb of life, effect,
   end, end of life, end point, ending, envoi, epilogue, eschatology,
   eternal rest, exit, expiration, expire, extinction, extinguishment,
   fall, fall asleep, fate, final solution, final summons,
   final twitch, final words, finale, finality, finger of death,
   finis, finish, go, go out, goal, going, going off, grave,
   hand of death, izzard, jaws of death, knell, last, last breath,
   last debt, last gasp, last muster, last rest, last roundup,
   last sleep, last things, last trumpet, last words, latter end,
   leaving life, loss of life, making an end, omega, part, parting,
   pass, pass away, pass on, pass over, passing, passing away,
   passing over, payoff, period, perish, perishing, peroration,
   put off mortality, quietus, quit this world, release, resolution,
   rest, resting place, return to dust, reward, sentence of death,
   shades of death, shadow of death, silence, sleep, somatic death,
   stop breathing, stoppage, stopping place, succumb,
   summons of death, swan song, term, terminal, termination, terminus,
   up and die, windup, yield the ghost