1) Type of arch 
An arcosolium is an arched recess used as a place of entombment. The word is from Latin arcus, `arch`, and solium, `sill` (literally: `place of state`). Early arcosolia were carved out of the living rock in catacombs. In the very earliest of these, the arched recess was cut to ground level. Then a low wall would be built in the front, leavin...
arcosolium 1. A type of tomb in Roman catacombs as well as Pagan and Christian burial places, consisting of an arched cell or niche, with a semi-circular vaulted ceiling. 2. In Roman catacombs, an arched recess for a sarcophagus.
(in Roman catacombs) an arched recess for a sarcophagus.