homology
This term is primarily used in biological and mathematical contexts to describe structural correspondence. In biology, it suggests a deep evolutionary link where different organs evolved from a single ancestral prototype, regardless of whether those organs now serve different functions. It is a technical term of precision, devoid of the casual connotations of similarity or likeness.
In mathematics, specifically topology, the term shifts toward a study of holes and connectivity within a space. The feeling of the word is clinical and analytical, used to categorize systemic patterns rather than superficial appearances. It implies a hidden, underlying architecture that binds two seemingly different things together.
Countable when referring to a specific instance of shared structure between two species. Uncountable when referring to the general biological principle of structural correspondence.