In engineering, a requirement is a condition that must be satisfied for the output of a work effort to be acceptable. It is an explicit, objective, clear and often quantitative description of a condition 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.
Requirements are used in many engineering fields including engineering design, system 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.