Usually a slightly sunken arcade or barrel vault creating a long walkway or storage area. Architecturally, they often also function as buttressing for larger, adjacent structures. Often lit by openings piercing the upper part of the vault.
In Ancient Roman architecture a cryptoporticus (from Latin crypta and porticus) is a covered corridor or passageway. The cryptoporticus is often at ground level or partially underground, and supports a structure such as a forum or Roman villa, in which case it served as basis villae. It is often vaulted and lit by openings in the vault.