From WordNet (r) 3.0 (2006) [wn]:
grotto
n 1: a small cave (usually with attractive features) [syn:
grotto, grot]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]