1. a literary or scientific institution, club, or reading room, especially one open to the public
Usage: often capitalized when referring to a specific institution; chiefly historical or formal
2. a building or hall used for lectures, readings, or cultural events
Usage: often capitalized when part of a proper name

Examples

• The Boston Athenaeum is one of the oldest membership libraries in the United States.
• Members of the athenaeum gathered weekly to discuss philosophy and literature.
• The athenaeum hosted a series of public lectures on scientific discoveries.
• She spent her afternoons reading in the quiet athenaeum near her home.
• The town's athenaeum served as a cultural center for the community.