Requirements

In product development and process optimization, a requirement is a singular documented physical and functional need that a particular design, product or process must be able to perform. It is most commonly used in a formal sense in systems engineering, software engineering, or enterprise engineering. It is a statement that identifies a necessary attribute, capability, characteristic, or quality of a system for it to have value and utility to a customer, organization, internal user, or other stakeholder. A requirement specification (often imprecisely referred to as the spec, because there are different sorts of specifications) refers to an explicit set of requirements to be satisfied by a material, design, product, or service.

In the classical engineering approach, sets of requirements are used as inputs into the design stages of product development. Requirements are also an important input into the verification process, since tests should trace back to specific requirements. Requirements show what elements and functions are necessary for the particular project. This is reflected in the waterfall model of the software life-cycle. However, when iterative methods of software development or agile methods are used, the system requirements are incrementally developed in parallel with design and implementation.

The term requirement has been in use in the software engineering community since at least the 1960s.

According to the Business Analysis Body of Knowledge (BABOK), a requirement is:

Requirements can be said to relate to two fields:

Product and process requirements are closely linked; a product requirement could be said to specify the automation required to support a process requirement while a process requirement could be said to specify the activities required to support a product requirement. For example, a maximum development cost requirement (a process requirement) may be imposed to help achieve a maximum sales price requirement (a product requirement); a requirement that the product be maintainable (a product requirement) often is addressed by imposing requirements to follow particular development styles (e.g., object-oriented programming), style-guides, or a review/inspection process (process requirements).

...
Wikipedia