Source : Webster's Revised Unabridged Dictionary (1913)

Grotto \Grot"to\, n.; pl. {Grottoes}. [Formerly grotta, fr. It.
   grotta, LL. grupta, fr. L. crypta a con cealed subterranran
   passage vault, cavern, Gr. ?, fr. ? concealed, fr. ? to
   conceal. Cf. {Grot}, {Crypt}.]
   A natural covered opening in the earth; a cave; also, an
   artificial recess, cave, or cavernlike apartment.

Source : WordNet®

grotto
     n : a small cave (usually with attractive features) [syn: {grot}]
     [also: {grottoes} (pl)]