Source : Webster's Revised Unabridged Dictionary (1913)

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.