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]