The Collaborative International Dictionary of English v.0.48:

Foundling \Found"ling\, n. [OE. foundling, fundling; finden to
   find + -ling; cf. f["u]ndling, findling. See Find, v. t.,
   and -ling.]
   A deserted or exposed infant; a child found without a parent
   or owner.
   [1913 Webster]

   Foundling hospital, a hospital for foundlings.
      [1913 Webster]

WordNet (r) 3.0 (2006):

foundling
    n 1: a child who has been abandoned and whose parents are
         unknown [syn: foundling, abandoned infant]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "foundling":
   bonus, buried treasure, castaway, castoff, derelict, discard,
   discovery, dogie, find, finding, flotsam, flotsam and jetsam,
   gravy, jetsam, junk, lagan, orphan, outcast, refuse, reject,
   rubbish, stray, trash, treasure trove, trouvaille, trove, waif,
   waifs, waifs and strays, wastrel, windfall, windfall money,
   windfall profit



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

FOUNDLING. A new-born child, abandoned by, its parents, who are unknown. The
settlement of, such a child is in the place where found.