porticus
A small room in a church, commonly forming extensions to the north and south sides of it, giving the building a cruciform plan, which may function as a chapel, rudimentary transept or burial place.
It remains therefore to speculate on the form of the church: whether we located north and south aisles beyond the nave or whether these are porticuses is not clear. The church could have been fully aisled, or it might have had shorter aisles extending to the east end, or just flanking porticuses.
An ancient Roman colonnade, arcade, or portico.
Mr. Codd agrees that this Roman porticus must have been an arcade, with the columns of which we have the bases attached to the square piers; this was a usual Roman construction, and the two bases at an angle (one of them partly inserted in the other) would belong to this plan, the wide distance apart of the bases also confirms the idea that they stood against arches.
Where it comes from
From Latin porticus. Doublet of porch, portego, and portico.
Adapted from the Wiktionary entry porticus (CC BY-SA 4.0).