The Collaborative International Dictionary of English v.0.48:

Omnibus \Om"ni*bus\, n. [L., for all, dat. pl. from omnis all.
   Cf. Bus.]
   1. A long vehicle, having seats for many people; a bus.
      [1913 Webster]

   Note: In the 1913 Webster the term omnibus was especially
         applied to, a vehicle with seats running lengthwise,
         used in conveying passengers short distances.
         [PJC]

   2. (Glass Making) A sheet-iron cover for articles in a leer
      or annealing arch, to protect them from drafts.
      [1913 Webster]

   3. (Printing) A volume containing collected and reprinted
      works of a single author or on a single theme.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

omnibus \om"ni*bus\, a.
   Pertaining to or dealing with a variety of topics at one
   time; as, omnibus legislation; an omnibus budget bill.
   [PJC]

   Omnibus bill, a legislative bill which provides for a
      number of miscellaneous enactments or appropriations.
      [Parliamentary Cant, U.S.]

   Omnibus box, a large box in a theater, on a level with the
      stage and having communication with it. [Eng.]
      --Thackeray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

omnibus
    adj 1: providing for many things at once; "an omnibus law"
    n 1: an anthology of articles on a related subject or an
         anthology of the works of a single author
    2: a vehicle carrying many passengers; used for public
       transport; "he always rode the bus to work" [syn: bus,
       autobus, coach, charabanc, double-decker, jitney,
       motorbus, motorcoach, omnibus, passenger vehicle]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "omnibus":
   Festschrift, absolute, across-the-board, aggregate, album, all,
   all-comprehensive, all-embracing, all-encompassing, all-inclusive,
   all-out, all-pervading, ana, analects, anthology, autobus,
   beauties, blanket, born, broad-based, bus, cab, canon,
   chartered bus, chrestomathy, clean, clear, collectanea,
   collected works, collection, compendious, compilation, complete,
   complete works, comprehensive, congenital, consummate, deep-dyed,
   delectus, diligence, double-decker, downright, dyed-in-the-wool,
   egregious, encyclopedic, entire, exhaustive, florilegium, flowers,
   garden, garland, global, gross, hack, hired car, holistic,
   inclusive, integral, integrated, intensive, jitney, mail coach,
   miscellanea, miscellany, motor coach, motorbus, omnipresent, one,
   one and indivisible, out-and-out, outright, over-all, panoramic,
   perfect, pervasive, photograph album, plain, plumb, post coach,
   posy, pure, quotation book, radical, regular, scrapbook, sheer,
   stage, stagecoach, straight, sweeping, symposium, synoptic, taxi,
   taxicab, thorough, thoroughgoing, through-and-through, total,
   ubiquitous, unconditional, universal, unmitigated, unqualified,
   unreserved, unrestricted, utter, veritable, whole, wholesale,
   without exception, without omission