What term describes a variable that cannot be changed?

The term that describes a variable that cannot be changed is "immutability." In programming, an immutable variable is one whose value cannot be altered after it is created. This concept is crucial in many programming languages, particularly functional programming paradigms, where immutability helps ensure that data remains consistent and predictable throughout its lifecycle.

For example, in languages such as Python, strings and tuples are immutable; once created, they cannot be modified. When you attempt to change an immutable object, a new object is generally created instead of altering the existing one. This behavior can lead to safer and often more efficient code because it helps avoid side effects, which are changes to objects that may unexpectedly affect other parts of the program.

The other terms, while related to the state of data, do not specifically denote the property of being unchanged. Thus, understanding immutability is key to grasping certain programming principles and paradigms that emphasize the importance of maintaining constant data states.