Final answer:
The term referring to a multinucleated mass of cells caused by the cytopathic effect of viral infection is a syncytium, which occurs when host cells fuse together.
Step-by-step explanation:
The term that refers to a multinucleated mass of cells that form when many host cells fuse together during a viral infection is a syncytium.
This process is a result of the cytopathic effect of viral infection, where viruses such as the human herpesvirus can cause cytopathic effects including cell abnormalities like the formation of multinucleated syncytia, inclusion bodies, and cell lysis.
Viral infections can also stimulate an immune response, where natural killer cells work in concert with cytotoxic T cells against virally infected cells.