The word is from Latin arcus, "arch", and solium, "throne" (literally "place of state") or post-classical "sarcophagus".
[dubious – discuss] Early arcosolia were carved out of the living rock in catacombs.
Then a low wall would be built in the front, leaving a trough (the cubiculum, "chamber") in which to place the body.
From the 13th century onwards, and continuing into the Renaissance, arcosolia were built above ground, particularly in the walls of churches.
In these post-Roman era recesses, which were built of brick and marble, the sarcophagi are usually separate from the arch.