The Collaborative International Dictionary of English v.0.48:

Sponsor \Spon"sor\ (-s[~e]r), n. [L., from spondere, sponsum, to
   engage one's self. See Spouse.]
   1. One who binds himself to answer for another, and is
      responsible for his default; a surety.
      [1913 Webster]

   2. One who at the baptism of an infant professes the
      Christian faith in its name, and guarantees its religious
      education; a godfather or godmother.
      [1913 Webster]

   3. A person who vouches for another as fit for some post or
      task; as, one needs two sponsors to be considered for
      membership.
      [PJC]

   4. A person or group that assumes financial responsibility
      for some activity, and may or may not participate in its
      organization and execution.
      [PJC]

   5. A person or organization, usually a commercial
      organization, which pays the cost of an activity, such as
      a radio or television broadcast, and in return is given
      the right to advertise itself or its products as part of
      the activity; as, now a word from our sponsor.
      [PJC]

WordNet (r) 3.0 (2006):

sponsor
    n 1: someone who supports or champions something [syn: patron,
         sponsor, supporter]
    2: an advocate who presents a person (as for an award or a
       degree or an introduction etc.) [syn: presenter, sponsor]
    v 1: assume sponsorship of [syn: sponsor, patronize,
         patronise]
    2: assume responsibility for or leadership of; "The senator
       announced that he would sponsor the health care plan"
    3: do one's shopping at; do business with; be a customer or
       client of [syn: patronize, patronise, shop, shop at,
       buy at, frequent, sponsor] [ant: boycott]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "sponsor":
   Maecenas, abettor, accept the responsibility, admirer, advocate,
   aficionado, angel, answer for, apologist, assure, attest, back,
   backer, bailsman, bankroll, be sponsor for, benefactor, bond,
   bondsman, buff, capitalize, certify, champion, confirm,
   countersecure, defender, dependence, encourager, endorse, endorser,
   ensure, exponent, fan, favorer, finance, financer, friend at court,
   fund, funder, godfather, godparent, grubstake, grubstaker,
   guarantee, guarantor, guaranty, insure, insurer, lover, mainpernor,
   mainstay, maintainer, meal ticket, mortgagor, paranymph, partisan,
   patron, patronize, pay for, promote, promoter, protagonist,
   provide for, refinance, reliance, second, seconder, sectary,
   secure, set up, sider, sign, sign for, stake, staker, stalwart,
   stand behind, stand sponsor for, stand up for, standby,
   subscribe to, subsidize, subsidizer, support, supporter, surety,
   sustainer, sympathizer, take the blame, take up, undersign,
   underwrite, underwriter, upholder, votary, warrant, warrantor,
   well-wisher



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

SPONSOR, civil law. He who intervenes for another voluntarily and without
being requested. The engagement which he enters into is only accessory to
the principal. Vide Dig. 17, 1, 18; Nov. 4, ch. 1 Code de Com. art. 158,
159; Code Nap. 1236 Wolff, Inst. Sec. 1556.