The Collaborative International Dictionary of English v.0.48:

Sacellum \Sa*cel"lum\, n.; pl. Sacella. [L., dim. of sacrum a
   sacred place.]
   (a) (Rom. Antiq.) An unroofed space consecrated to a
       divinity.
   (b) (Eccl.) A small monumental chapel in a church. --Shipley.
       [1913 Webster]