Design by contract, also known as contract programming, programming by contract and design-by-contract programming, is an approach for designing software. Wikipedia

A methodology for designing computer software Wikipedia Disambiguation

An approach to designing computer software that prescribes that software designers should define precise verifiable interface specifications for software components based upon the theory of abstract data types and the conceptual metaphor of a business contract. OmegaWiki

A software design approach that prescribes that software designers should define formal, precise and verifiable interface specifications ("contracts") for software components that extend the ordinary definition of abstract data types with preconditions, postconditions and invariants. Wiktionary