Requirement

In engineering, a requirement is a need that a particular item must satisfy for it to be acceptable.

Requirements are used in many engineering fields including engineering design, systems engineering, software engineering, enterprise engineering, product development, and process optimization.

Requirement is a relatively broad concept that can describe any necessary or desired function, attribute, capability, characteristic, or quality of a system for it to have value and utility to a customer, organization, user, or other stakeholder.

A requirement is an explicit, highly objective, clear and often quantitative description of a need to be satisfied by a material, design, product, or service.^[1]

A specification or spec is a set of requirements that is typically used by developers in the design stage of product development and by testers in their verification process.

With iterative and incremental development such as agile software development, requirements are developed in parallel with design and implementation. With the waterfall model, requirements are completed before design or implementation start.