Functional

Generally, functional refers to something able to fulfill its purpose or function.

Functional (mathematics) is a term applied to functions in mathematics, computer science, and Lagrangian mechanics.
Functional programming is used in computer science as well.
Functional form and functionalism (architecture) apply to architectural design.
Functional symptom is sometimes used in medicine to describe symptoms that have no organic basis, e.g. if they are a result of psychological or perceptual dysfunction.
Diatonic functionality describes a music term.