The Collaborative International Dictionary of English v.0.48:

grotto \grot"to\ (gr[o^]t"t[-o]), n.; pl. Grottoes
   (gr[o^]t"t[=o]z). [Formerly grotta, fr. It. grotta, LL.
   grupta, fr. L. crypta a concealed subterranean passage,
   vault, cavern, Gr. kry`pth, fr. krypto`s concealed, fr.
   kry`ptein to conceal. Cf. Grot, Crypt.]
   A natural covered opening in the earth; a cave; also, an
   artificial recess, cave, or cavernlike apartment.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grotto
    n 1: a small cave (usually with attractive features) [syn:
         grotto, grot]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "grotto":
   abri, antre, bunker, burrow, cave, cavern, cove, dugout, foxhole,
   grot, hole, lair, sewer, subterrane, subterranean, subway, tunnel,
   warren