An inlay is a woodworking term where an ornament is embedded in a host material which sits flush with it's surface e.g. the ivory is inlaid in the maple. In general, an "inlay pocket" is cut in the host material, then the inlay itself is cut and placed into the inlay pocket.
In cue sports, inlays are common in higher-end pool cues and pool tables. Before computers took over the production of inlays in pool cues and billiard tables, they were done by hand with a skill called "Marquetry".
Pool Cue Inlay
The ornamentation in a pool cue is often made by inlaying exotic materials into the wood of the butt portion of the cue. The cue maker (or computer/CNC machine) makes inlay pockets into which another material is placed.
High end hand-made and production pool cues often feature one or more inlays and are so expensive and valuable because of the time and skill required to do an inlay by hand. The value of a pool cue is often closely tied to the quality and quantity of its inlays.
Here is a video by Bob Dzuricky (of DZ Cues) cutting inlay pockets in a new pool cue:
And here is the process of cutting the inlay material (in this case, ivory).
Materials such as ebony and ivory are quite commonly used.
These are photos of a French pool cue from the early 1800's which features intricate marquetry inlay all done by hand.
Pool Table with Inlay
Most folks would be familiar with the pool table's diamonds or "sights" on the rails. They are typically "inlaid" into the wood of the rails.