Linked from
The 11 pages that link to Bound variable, each with the reason it gives.
QuantifierRelated: Quantifiers bind variables to the objects over which they range.
Free variableCompared with: It contrasts with a free occurrence because a binder fixes its scope.
Domain of discourseRelated: A quantifier binds a variable and ranges it over domain objects.
Skolem normal formRelated: Skolemization replaces bound existential variables with fresh symbols.