What is the term for a house's second or tradesman's entrance, usually at the rear leading out of the peristyle?

The term for a house's second or tradesman's entrance, typically located at the rear leading out of the peristyle, is referred to as "Posticum." In Roman architecture, the posticum served a functional purpose by allowing access for service personnel and goods without disrupting the main living areas of the house. This design element was practical for maintaining privacy and cleanliness within the household while facilitating the movement of tradespeople and deliveries.

The other terms listed refer to different structures or features within Roman architecture and society. The "Gartibulum" is not a recognized term in this context and may be a misunderstanding or misrepresentation of another architectural term. "Caupona" refers specifically to an inn or tavern, a completely different type of space used for lodging or food services. "Arca" generally refers to a chest or a box, which does not relate to entrances in buildings. Thus, the proper term for the described entrance in the context of Roman domestic architecture is indeed "Posticum."