An "onlay" is a form of dental restoration used to repair a decayed, chipped, cracked, or otherwise damaged tooth. An onlay corrects damage extending to a "cusp" (the raised points on the biting surface), in contrast to an inlay, which is used to repair only that portion of the tooth between these cusps.
Generally speaking, there are two different types of dental restorations: "direct" and "indirect." For example, a "filling" is a type of direct restoration, because the material used to repair the tooth undergoes its hardening process while in "direct" contact with the tooth, and *inside* the mouth.
Unlike a filling, where a malleable material is placed into the tooth, and allowed to harden, an onlay is an *already* hardened material (like a crown) that is joined to the tooth by bonding or cement. Because the material used to repair the tooth is created *outside the mouth*, either in a lab by a dental technician, or via a computer-controlled milling machine, the process is considered an "indirect" restoration.
In all, there are five surfaces of the tooth eligible for restoration: the distal, occlusal, buccal, mesial, and lingual/palatal surfaces. With this dental procedure code, an onlay is made from any range of metals including: gold, silver and palladium, and placed on four or more of those surfaces.
To prepare for an onlay on four or more surfaces, a dentist will first remove any decayed or weakened areas of the tooth. Then, depending on the process used by your dentist, either a physical impression of the tooth will be made, or 3D imaging will be used to render a digital impression. Next, you may be fitted with a temporary onlay until the custom version is completed, or if your onlay can be fabricated on-site your dentist will proceed with the installation.