What is the difference between a unique constraint and a unique index?

A unique constraint can be used to guarantee that one or more column values are unique when one record is added to the base table. If a unique constraint has been given or defined above,
It is not possible for any operation to replace the copied values in these columns. Although unique, system-required indexes are used to enforce unique constraints, define unique constraints
There is a difference between creating a unique index. Even if both of these can enhance uniqueness, a unique index allows NULL values and is generally not used in indicative
Constraints. In other words, a unique constraint does not allow null values and can be used in a foreign key specification ("NULL" means that the column value is ambiguous and differs from other values
, including other null values).

What is the difference between a unique constraint and a unique index?