Which term is used by pathologists to describe a group of cells that can be seen only by a microscope?

The term "tumor focus" is specifically utilized by pathologists to refer to a localized area within a tumor that contains a group of cancer cells that can be identified exclusively under microscopy. This terminology emphasizes the distinct and confined nature of the cell group, which is crucial in diagnosing and analyzing tumors. The concept of a tumor focus helps in understanding the extent of the disease and can influence treatment decisions as it reflects localized tumor characteristics.

Other terms listed in the options have different meanings in the context of pathology. For instance, "focal tumor" may refer to specific tumors that exhibit localized growth but doesn't specifically highlight the microscopic visibility aspect. "Overlapping tumor" and "contiguous tumor" address spatial relationships or growth patterns of tumors rather than the microscopic identification of specific cell groups. "Tumor focus" is the term that most accurately captures the microscopic nature of the cells in question.