obligate

This term carries a heavy sense of necessity and constraint, often appearing in legal or formal contexts to describe a binding requirement. It differs from "force" by implying a structured duty or a contractual agreement rather than raw power or coercion.

In biological sciences, the adjective form describes an absolute requirement for survival. This creates a sharp distinction between "obligate" and "facultative" organisms, where the former has no alternative means of existence and the latter possesses flexibility in its environmental needs.