A Requirement is a prioritized demand placed on the project by one of its interested parties. Often it is the buyer the user or the management that have Requirements to the scope or results of the projects. But it can also technical requirements that appears because of constraints in the physical equipment or the algorithmic solutions chosen.
It is recommended to build a Requirement hierarchy by relating one Requirement to the sub requirements that composes it.
Requirement can be modelled in a Requirement Model.