n. A shrine open to the sky, sometimes used for sacrificial purposes, or in honor of the divine
the GNU version of the Collaborative International Dictionary of English
n. An unroofed space consecrated to a divinity.
n. A small monumental chapel in a church.
The Century Dictionary and Cyclopedia
n. In Roman antiquity, a small inclosed space without a roof, consecrated to some deity, containing an altar, and sometimes also a statue of the god.
Word Usage
"In that case, the open front and the niche in the back wall could suggest that one is dealing here with a kind of sacellum (or shrine) dedicated to the cult of Marcus Aurelius, to whom the whole complex was dedicated."