An Immutable Set is a data structure that contains a collection of unique elements that cannot be changed after it is created. This means you cannot add, remove, or modify elements within the set. Immutable sets are useful in programming because they provide a stable collection of items that can be relied upon not to change, which can help prevent errors in code. In many programming languages, such as Python and Java, immutable sets are implemented to enhance performance and ensure data integrity. They are often used in situations where a fixed set of values is needed, such as in functional programming or when working with hash tables.