Definition of "Requirement"

Here is my definition of "requirement":

"A requirement specifies the least stringent condition that must hold to solve or avoid a prospect problem (problem that a prospective customer faces)."

Note that a requirement, under this definition:

1. Is possible, in principle, to test.
2. Does not specify a design or solution.
3. Correlates unambiguously to a problem that a prospective customer faces.

UPDATE: You can find a comprehensive model of requirements concepts here.