The Collaborative International Dictionary of English v.0.48:

Ostensible \Os*ten"si*ble\ ([o^]s*t[e^]n"s[i^]*b'l), a. [From L.
   ostensus, p. p. of ostendere to show, prop., to stretch out
   before; fr. prefix obs- (old form of ob-) + tendere to
   stretch. See Tend.]
   [1913 Webster]
   1. Capable of being shown; proper or intended to be shown.
      [R.] --Walpole.
      [1913 Webster]

   2. Outwardly appearing to be; shown to be; exhibited;
      apparent; evident.
      [1913 Webster +PJC]

   3. Declared; avowed; professed; pretended; -- often used as
      opposed to real or actual; as, an ostensible reason,
      motive, or aim. --D. Ramsay.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

ostensible
    adj 1: appearing as such but not necessarily so; "for all his
           apparent wealth he had no money to pay the rent"; "the
           committee investigated some apparent discrepancies"; "the
           ostensible truth of their theories"; "his seeming
           honesty" [syn: apparent(a), ostensible, seeming(a)]
    2: represented or appearing as such; pretended; "His ostensible
       purpose was charity, his real goal popularity" [syn:
       ostensible, ostensive]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "ostensible":
   Barmecidal, Barmecide, airy, alleged, apparent, apparitional,
   appearing, arrant, autistic, avowed, blatant, bold, chimeric,
   claimed, colorable, colored, conspicuous, deceptive, delusional,
   delusionary, delusive, delusory, dereistic, dreamlike, dreamy,
   erroneous, fallacious, false, fantastic, flagrant, gilded, glaring,
   hanging out, hypocritical, illusional, illusionary, illusive,
   illusory, imaginary, in name only, in relief, in the foreground,
   meretricious, misleading, notable, noticeable, notorious,
   obtrusive, outstanding, outward, phantasmagoric, phantasmal,
   phantom, plausible, pretended, pretexted, professed, prominent,
   pronounced, purported, salient, seeming, self-deceptive,
   self-deluding, so-called, specious, spectral, staring,
   stark-staring, sticking out, striking, superficial, supposed,
   supposititious, surface, tinsel, to the eye, unactual, unfounded,
   unreal, unsubstantial, visible, visionary