The Collaborative International Dictionary of English v.0.48:

ambrosia \am*bro"sia\ ([a^]m*br[=o]"zh[.a]
   or[a^]m*br[=o]"zh[i^]*[.a]; 277), n. [L. ambrosia, Gr.
   'ambrosi`a, properly fem. of 'ambro`sios, fr. 'a`mbrotos
   immortal, divine; 'a priv. + broto`s mortal (because it was
   supposed to confer immortality on those who partook of it).
   broto`s stands for mroto`s, akin to Skr. m[.r]ita, L.
   mortuus, dead, and to E. mortal.]
   1. (Myth.)
      (a) The fabled food of the gods (as nectar was their
          drink), which conferred immortality upon those who
          partook of it.
      (b) An unguent of the gods.
          [1913 Webster]

                His dewy locks distilled ambrosia. --Milton.
          [1913 Webster]

   2. A perfumed unguent, salve, or draught; something very
      pleasing to the taste or smell. --Spenser.
      [1913 Webster]

   3. Formerly, a kind of fragrant plant; now (Bot.), a genus of
      plants, including some coarse and worthless weeds, called
      ragweed, hogweed, etc.
      [1913 Webster]

   4. (Zool.) The food of certain small bark beetles, family
      Scolytidae believed to be fungi cultivated by the
      beetles in their burrows.
      [Webster 1913 Suppl.]

   5. A dessert made from shredded coconuts and oranges,
      sometimes including other ingredients such as marshmallow.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

ambrosia
    n 1: a mixture of nectar and pollen prepared by worker bees and
         fed to larvae [syn: beebread, ambrosia]
    2: any of numerous chiefly North American weedy plants
       constituting the genus Ambrosia that produce highly
       allergenic pollen responsible for much hay fever and asthma
       [syn: ragweed, ambrosia, bitterweed]
    3: fruit dessert made of oranges and bananas with shredded
       coconut
    4: (classical mythology) the food and drink of the gods; mortals
       who ate it became immortal [syn: ambrosia, nectar]