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  Statement that translates or expresses a need and its associated constraints and conditions.
NIST SP 800-160v1r1 from ISO/IEC/IEEE 29148:2018

  A condition or capability that must be met or possessed by a system or system element to satisfy an agreement, standard, specification, or other formally imposed documents.
NIST SP 800-160v1r1 from IEEE Std. 730-2014

  A need or expectation that is stated, generally implied or obligatory.
NIST SP 800-188

  A phase of the Development Process wherein the top level definition of the functionality of the system is produced.
NISTIR 5153 under Requirements