Definitions for "Unique Constraint"
a restriction on a data column in which all values must be unique
A constraint that enforces entity integrity on a non-primary key. It ensures no duplicate values are entered in the columns on which the constraint is placed. NULL values are allowed as long as rows are unique in the table.
A restriction on a column or set of columns requiring that all non-null values are different. A table can have multiple unique constraints. See also: foreign key constraint, primary key constraint, constraint.