A slave owned by another slave is known as what?

A slave owned by another slave is referred to as a "vicarius." In ancient Roman society, the term "vicarius" describes a situation where one slave is subordinated to another, functioning in a similar capacity as a sort of helper or assistant. This arrangement was not uncommon in Roman households, where hierarchies among slaves existed.

The other terms provided have distinct meanings. A "libertus" is a freedman, someone who was formerly a slave but has been granted freedom. "Patronus" denotes a patron, typically an individual who has granted freedom to a slave and has obligations of support and protection to that freedman. Lastly, a "titulus" refers to a deed or a label, particularly in legal terms, and does not relate to the ownership structure among slaves. Thus, "vicarius" specifically aligns with the context of a slave being owned by another slave.