Software requirements[1] for a system are the description of what the system should do, the service or services that it provides and the constraints on its operation. The IEEE Standard Glossary of Software Engineering Terminology defines a requirement as:[2]
The activities related to working with software requirements can broadly be broken down into elicitation, analysis, specification, and management.[3]
Note that the wording Software requirements is additionally used in software release notes to explain, which depending on software packages are required for a certain software to be built/installed/used.[1]