Requirement

[Britney]

That which is required; an imperative or authoritative
command; an essential condition; something needed or necessary; a need.

[Marquetta]

A requirement is a subsets of the problem domain. This can be compared to the design which is a subset of the machine and focuses on the solution rather than problem. The union of the two is done through the specification.

1a) Must distinguish preferences, priorities

[Mark Zuckerberg]

A must have. Needed to continue.

[Becky]

A thing that is needed or wanted.