According to the BABOK Guide, a requirement is a usable representation of a need. A requirement does not cease to be a requirement once it has been satisfied; it remains a requirement as long as the need exists and the solution is in use. Requirements may change over time due to changes in the business environment, stakeholder needs, or solution performance1. Reference: 1: BABOK Guide, version 3, section 3.2.1, page 34.