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A formal or informal methodology for designing, creating, and maintaining software (including code built into hardware).
NIST SP 800-218 under Software Development Life Cycle
The scope of activities associated with a system, encompassing the system’s initiation, development and acquisition, implementation, operation and maintenance, and ultimately its disposal that instigates another system initiation.
NISTIR 7622 under System Development Life Cycle from CNSSI 4009