Requirement
A statement of what a system must do or a characteristic it must have. Requirements can be Functional (behavior) or Non-Functional (quality criteria like speed, security).
Impact
Ambiguous requirements are the root cause of most project failures. You cannot build what you cannot define.
Weinto take
We prefer User Stories over Requirements Documents. A 500-page requirement spec is a lie; nobody reads it and it's out of date the moment it's printed. A User Story ("As a user, I want X so that Y") is a placeholder for a conversation, keeping the focus on user value.