In the context of coping stones, 'expansion joint' could be an option but it's hard to be absolutely sure about this without further context.

Example:

Almost all in ground pools that are thoughtfully constructed will have an expansion joint. This is usually the gap between the coping stones that surround the pool from the decking. The joint is usually filled with caulk or some kind of sealant that lets the materials on both sides of it move without causing serious damage to either.

If the concrete were poured right up to the coping stones without any room left for a pool expansion joint, you would have serious cracking when the deck expands during warm weather after having contracted in the cold of the winter.
https://sunplay.com/blogs/posts/pool-expansion-joint

Depending on the actual context, 'control joint' could be another option.

Example:

Control joints traverse the slab, typically the deck, especially in pool projects. These joints are there to control the location and appearance of cracks, which are often due to concrete shrinkage and other factors related to expansion and contraction. They can also reduce the impact cracks have on the "performance" of the slab or other type of structure, even though they are primarily there for aesthetic reasons.

Expansion joints accommodate normal movement of adjoining concrete structures. They are the critical buffer between pool decks on one side, and the tile line, coping and mortar bed on the other. In areas with expansive soil, including most of California where I work, expansion joints are particularly important, but the same can be said for areas with freeze/thaw conditions.
https://aquamagazine.com/features/mind-the-gaps.html