What is the definition of solarium?

An apartment freely exposed to the sun; anciently, an apartment or inclosure on the roof of a house; in modern times, an apartment in a hospital, used as a resort for convalescents., Any one of several species of handsome marine spiral shells of the genus Solarium and allied genera. The shell is conical, and usually has a large, deep umbilicus exposing the upper whorls. Called also perspective shell.