An inlay is an indirect restoration fabricated outside the mouth and then cemented into a prepared cavity. It is typically made from materials like cast gold or porcelain, which provide superior strength and durability compared to direct restorations. Unlike direct fillings such as composite or amalgam, which are placed and shaped directly in the tooth, an inlay fits precisely within the contours of the cavity without covering the cusps. Cast gold restoration is a classic example of an inlay due to its excellent fit, biocompatibility, and wear resistance. This distinguishes it clearly from direct composite, amalgam, or glass ionomer restorations.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.