Subtype

A subtype is a category that represents a more specific form of a broader category. Subtyping is the relation by which a member of a subtype can be treated as a member of its supertype, while also possessing additional properties or restrictions. The term is used across disciplines, including mathematics and logic, computer science, biology, medicine, and linguistics.

In type theory and programming, a type B is a subtype of type A if every value of B can be used wherever a value of A is expected. This enables polymorphism and code reuse. Subtyping is distinct from inheritance, though related, and may involve rules about how function types vary in their inputs and outputs (covariance and contravariance).

In data modeling and software design, subtypes appear in generalization–specialization hierarchies with supertype and subtype labels. Subtypes may be disjoint (a value belongs to one subtype only) or overlapping (a value may belong to several subtypes). Example: supertype Employee with subtypes SalariedEmployee and HourlyEmployee; subtypes may introduce extra attributes.

In biology and medicine, a subtype denotes a population or form within a larger category. In biology, this is often called a subspecies or variety; in medicine, disease subtypes (for example, hormone receptor–positive or HER2-positive breast cancer) reflect variation that affects prognosis or treatment.

In linguistics and information organization, subtype distinctions help classify words, phrases, or data records into more specific classes, aiding retrieval and analysis.