An onlay is a type of cusp-covering dental restoration that is made in a laboratory by a skilled technician, or by a computer controlled milling machine. Onlays can be made of ceramic, composite resin, gold, titanium or other metals. Metallic onlays are generally cemented (glued) onto the tooth, at a separate appointment from the tooth preparation appointment.
Ceramic and composite resin onlays are generally bonded (fused with adhesive) onto the tooth, either at the preparation appointment (if CAD/CAM is available for ceramic onlay production) or at a second appointment (more commonly the case). If you need two appointments, it will often mean having to numb the tooth at both appointments, and having a temporary restoration placed between appointments.
Onlays are like crowns, in that they afford protection against cracking to the tooth; but they’re more conservative in the amount of natural tooth structure that needs to be removed to make room for them. They’re also similar to inlays, except that they cover at least one of the pointed chewing cusps of the tooth.