The Collaborative International Dictionary of English v.0.48:

Cenotaph \Cen"o*taph\ (s[-e]n"[-o]*t[.a]f), n. [Gr. kenota`fion;
   keno`s empty + ta`fos burial, tomb: cf. F. c['e]notaphe.]
   An empty tomb or a monument erected in honor of a person who
   is buried elsewhere. --Dryden.
   [1913 Webster]

         A cenotaph in Westminster Abbey.         --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cenotaph
    n 1: a monument built to honor people whose remains are interred
         elsewhere or whose remains cannot be recovered [syn:
         cenotaph, empty tomb]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "cenotaph":
   arch, barrow, beehive tomb, bone house, boundary stone, box grave,
   brass, burial, burial chamber, burial mound, bust, cairn,
   catacombs, charnel house, cist, cist grave, column, cromlech,
   cross, crypt, cup, cyclolith, deep six, dokhma, dolmen, footstone,
   grave, gravestone, headstone, hoarstone, house of death,
   inscription, last home, long home, low green tent, low house,
   marker, mastaba, mausoleum, megalith, memento, memorial,
   memorial arch, memorial column, memorial statue, memorial stone,
   menhir, monolith, monstrance, monument, mound, mummy chamber,
   narrow house, necrology, obelisk, obituary, ossuarium, ossuary,
   passage grave, pillar, pit, plaque, prize, pyramid, reliquary,
   remembrance, resting place, ribbon, rostral column, sepulcher,
   shaft, shaft grave, shrine, stela, stone, stupa, tablet,
   testimonial, tomb, tombstone, tope, tower of silence, trophy,
   tumulus, vault



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CENOTAPH. An empty tomb. Dig. 11, 7, 42.