Scope
The extent of the area or subject matter that something deals with or to which it is relevant. In projects, it defines what is included and, crucially, what is excluded.
Impact
Scope Creep (uncontrolled changes in scope) is a common cause of project failure.
Weinto take
We treat Scope as the Variable. In the Iron Triangle (Time, Cost, Scope), we prefer to fix Time and Cost (budget) and vary Scope to fit. This ensures we always ship something on the deadline, even if it excludes some features.