A (usually) antique item, such as a clock, radio, sewing machine, spinning wheel, tool,car parts, etc, that may or may not work, kept strictly as a display item. More often than not the item does not work but the owner has no intention of repairing it, it just sits there.
If it is sometimes used and is not strictly a decoration it is NOT a shelf queen.
"Did Joe ever get that Sparton Bluebird working?"
"No, it's just a shelf queen for his mantel. Thousands of dollars for a non-working radio, just for display!"