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A program variable is a named binding or storage abstraction used to supply a value during execution of a computer program. Its precise meaning belongs to the programming language semantics: some languages describe mutable storage locations, while others emphasize names associated with values. The current semantic environment and program state determine what an occurrence denotes. A mathematical variable and a program variable consequently need not obey the same rules.

For example, in Python, executing x = 3 followed by x = x + 1 first evaluates the right-hand program expression using the old binding and then binds x to the resulting integer 4. This variable assignment is a change in the program state, not the equation $x=x+1$.^[ref]

Binding a second name does not necessarily copy the underlying data. After a = list() and b = a, executing b.append(7) changes the list also reached through a. By contrast, b = list() rebinds only b. Distinguishing changes to data from changes to names makes shared mutable structures easier to reason about.^[ref]