Requirement engineering is the process of gathering, analyzing, and documenting user needs and expectations for a software system.

Requirement elicitation is the process of gathering user needs and expectations. Common techniques include interviewing stakeholders, observing user behavior, and analyzing documents.

A requirement specification is a document that describes the software system in detail. It serves as a contract between the stakeholders and the development team, and it facilitates communication among stakeholders.

Requirements can be classified into different types, including functional requirements, non-functional requirements, and business requirements.

Functional requirements describe what the software system should do, while non-functional requirements describe how the software system should perform. Functional requirements are mandatory, while non-functional requirements are optional. Functional requirements are typically expressed in natural language, while non-functional requirements are typically expressed in mathematical notation.

A use case is a description of a typical interaction between a user and the software system. It identifies the actors and their goals, and it helps to identify the functional requirements of the software system.

Requirement analysis is the process of understanding and refining the requirements gathered during requirement elicitation. Common techniques include use case analysis, scenario analysis, and domain analysis.

A requirement traceability matrix is a tool that helps to trace the requirements from the requirement specification to the design and implementation, the test cases, and the user documentation.

Requirement engineering is a complex and challenging process. Common challenges include gathering complete and accurate requirements, managing changing requirements, and communicating requirements effectively among stakeholders.

Changing requirements are a reality in software development. The best way to manage them is to document them, update the requirement specification, communicate them to the stakeholders, and get their approval.

Requirement validation is the process of ensuring that the requirements are complete, accurate, consistent, and feasible. Common techniques include peer review, prototyping, and user acceptance testing.

A requirement management tool is a software tool that helps to manage the requirements throughout the software development lifecycle. It provides a central repository for all requirements, facilitates communication among stakeholders, and helps to ensure that the requirements are complete, accurate, consistent, and feasible.

Requirement management tools can be implemented using a variety of technologies, including spreadsheets, databases, and dedicated requirement management tools.

To ensure that the requirements are met, it is important to test the software system thoroughly, get feedback from the stakeholders, and review the software system against the requirements.

Requirement engineering is an iterative process that involves gathering, analyzing, and refining the requirements until they are complete, accurate, consistent, and feasible.