Requirement

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.
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.

Origins of term

The term requirement has been in use in the software engineering community since at least the 1960s.
According to the Guide to the Business Analysis Body of Knowledge® version 2 from IIBA, a requirement is:

A condition or capability needed by a stakeholder to solve a problem or achieve an objective.
A condition or capability that must be met or possessed by a solution or solution component to satisfy a contract, standard, specification, or other formally imposed documents.
A documented representation of a condition or capability as in or.

This definition is based on IEEE 610.12-1990: IEEE Standard Glossary of Software Engineering Terminology.

Product versus process requirements

Requirements can be said to relate to two fields:

Product requirements prescribe properties of a system or product.
Process requirements prescribe activities to be performed by the developing organization. For instance, process requirements could specify the methodologies that must be followed, and constraints that the organization must obey.

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 may be imposed to help achieve a maximum sales price requirement ; a requirement that the product be maintainable often is addressed by imposing requirements to follow particular development styles, style-guides, or a review/inspection process.

Types of requirements

Requirements are typically classified into types produced at different stages in a development progression, with the taxonomy depending on the overall model being used. For example, the following scheme was devised by the International Institute of Business Analysis in their Business Analysis Body of Knowledge.
; Architectural requirements
; Business requirements
; User requirements
; Functional requirements
; Quality-of-service requirements
; Implementation requirements
;Regulatory requirements

Characteristics of good requirements

The characteristics of good requirements are variously stated by different writers, with each writer generally emphasizing the characteristics most appropriate to their general discussion or the specific technology domain being addressed. However, the following characteristics are generally acknowledged.

Characteristic	Explanation
Unitary	The requirement addresses one and only one thing.
Complete	The requirement is fully stated in one place with no missing information.
Consistent	The requirement does not contradict any other requirement and is fully consistent with all authoritative external documentation.
Non-Conjugated	The requirement is atomic, i.e., it does not contain conjunctions. E.g., "The postal code field must validate American and Canadian postal codes" should be written as two separate requirements: "The postal code field must validate American postal codes" and "The postal code field must validate Canadian postal codes".
Traceable	The requirement meets all or part of a business need as stated by stakeholders and authoritatively documented.
Current	The requirement has not been made obsolete by the passage of time.
Unambiguous	The requirement is concisely stated without recourse to technical jargon, acronyms, or other esoteric verbiage. It expresses objective facts, not subjective opinions. It is subject to one and only one interpretation. Vague subjects, adjectives, prepositions, verbs and subjective phrases are avoided. Negative statements and compound statements are avoided.
Specify Importance	Many requirements represent a stakeholder-defined characteristic the absence of which will result in a major or even fatal deficiency. Others represent features that may be implemented if time and budget permits. The requirement must specify a level of importance.
Verifiable	The implementation of the requirement can be determined through basic possible methods: inspection, demonstration, test or analysis.

There are many more attributes to consider that contribute to the quality of requirements. If requirements are subject to rules of data integrity then accuracy/correctness and validity/authorization are also worthy attributes. Traceability confirms that the requirement set satisfies the need.
To the above some add Externally Observable, that is, the requirement specifies a characteristic of the product that is externally observable or experienced by the user. Such advocates argue that requirements that specify internal architecture, design, implementation, or testing decisions are probably constraints, and should be clearly articulated in the Constraints section of the Requirements document. The contrasting view is that this perspective fails on two points. First, the perspective does not recognize that the user experience may be supported by requirements not perceivable by the user. For example, a requirement to present geocoded information to the user may be supported by a requirement for an interface with an external third party business partner. The interface will be imperceptible to the user, though the presentation of information obtained through the interface certainly would not. Second, a constraint limits design alternatives, whereas a requirement specifies design characteristics. To continue the example, a requirement selecting a web service interface is different from a constraint limiting design alternatives to methods compatible with a Single Sign-On architecture.

Verification

All requirements should be verifiable. The most common method is by test. If this is not the case, another verification method should be used instead.
Certain requirements, by their very structure, are not verifiable. These include requirements that say the system must never or always exhibit a particular property. Proper testing of these requirements would require an infinite testing cycle. Such requirements must be rewritten to be verifiable. As stated above all requirements must be verifiable.
Non-functional requirements, which are unverifiable at the software level, must still be kept as a documentation of customer intent. However, they may be traced to process requirements that are determined to be a practical way of meeting them. For example, a non-functional requirement to be free from backdoors may be satisfied by replacing it with a process requirement to use pair programming. Other non-functional requirements will trace to other system components and be verified at that level. For example, system reliability is often verified by analysis at the system level. Avionics software with its complicated safety requirements must follow the DO-178B development process.
Activities that lead to the derivation of the system or software requirements. Requirements engineering may involve a feasibility study or a conceptual analysis phase of the project and requirements elicitation and requirements analysis, analysis, specification and validation.
Requirements are prone to issues of ambiguity, incompleteness, and inconsistency. Techniques such as rigorous inspection have been shown to help deal with these issues. Ambiguities, incompleteness, and inconsistencies that can be resolved in the requirements phase typically cost orders of magnitude less to correct than when these same issues are found in later stages of product development. Requirements analysis strives to address these issues.
There is an engineering trade off to consider between requirements which are too vague, and those which are so detailed that they

take a long time to produce - sometimes to the point of being obsolete once completed
limit the implementation options available
are costly to produce

Agile approaches evolved as a way of overcoming these problems, by baselining requirements at a high-level, and elaborating detail on a just-in-time or last responsible moment basis.

Documenting requirements

Requirements are usually written as a means for communication between the different stakeholders. This means that the requirements should be easy to understand both for normal users and for developers. One common way to document a requirement is stating what the system must do. Example: 'The contractor must deliver the product no later than xyz date.' Other methods include use cases and user stories.