The Collaborative International Dictionary of English v.0.48:

Liturgy \Lit"ur*gy\ (l[i^]t"[u^]r*j[y^]), n.; pl. Liturgies
   (l[i^]t"[u^]r*j[i^]z). [F. liturgie, LL. liturgia, Gr.
   leitoyrgi`a a public service, the public service of God,
   public worship; (assumed) le`i:tos, lei^tos, belonging to the
   people, public (fr. lao`s, lew`s, the people) + the root of
   'e`rgon work. See Lay, a., and Work.]
   An established formula for public worship, or the entire
   ritual for public worship in a church which uses prescribed
   forms; a formulary for public prayer or devotion. In the
   Roman Catholic Church it includes all forms and services in
   any language, in any part of the world, for the celebration
   of Mass.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Liturgy
    n 1: a Christian sacrament commemorating the Last Supper by
         consecrating bread and wine [syn: Holy Eucharist,
         Eucharist, sacrament of the Eucharist, Holy
         Sacrament, Liturgy, Eucharistic liturgy, Lord's
         Supper]
    2: a rite or body of rites prescribed for public worship

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "liturgy":
   Mass, baccalaureate service, bedtime prayer, camp meeting,
   celebration, ceremonial, ceremony, church, church service,
   commencement, compline, convocation, devotions, divine service,
   duty, empty formality, evening devotions, evensong, exercise,
   exercises, form, form of worship, formal, formality, formula,
   formulary, function, graduation, graduation exercises, holy rite,
   inaugural, inauguration, initiation, institution, lauds, matins,
   meeting, mode of worship, morning devotions, mummery, mystery,
   night song, none, nones, novena, observance, office,
   order of worship, ordinance, performance, practice, praise meeting,
   prayer, prayer meeting, prayers, prescribed form, prime,
   prime song, public worship, religious ceremony, revival,
   revival meeting, rite, rite de passage, rite of passage, ritual,
   ritual observance, rituality, sacrament, sacramental, service,
   sext, solemnity, solemnization, tent meeting, tierce, undersong,
   vesper, vespers, vigils, watch meeting, watch night,
   watch-night service