Immutability is the inability to modify data after it has been created. Modifications are instead made by copying the data. A property of immutable data is that it is *referentially transparent*.
Immutability is the inability to modify a variable after it is has been created.
It is a pattern found in many branches of programming; immutable objects are used widely within object oriented languages (such as Python's
str type, Java's
Integer type, .NET's
System.String, etc.), functional programming (esp. Haskell and other pure languages), and other paradigms.