What does a 'volatile' variable signify in programming?

A 'volatile' variable is used in programming, particularly in multithreading environments, to indicate that its value may be altered by different threads or external processes outside the immediate context of the variable's scope. This declaration informs the compiler and the runtime that it should not cache the variable’s value in registers or optimize away the reading or writing of this variable, as its value could change at any moment.

The significance of marking a variable as 'volatile' lies in ensuring that the most up-to-date value is read whenever it is accessed, rather than relying on potentially outdated cached values. This is crucial in scenarios where different threads may change the state of the variable simultaneously, preventing inconsistencies and ensuring that changes made by one thread are visible to other threads. Therefore, it provides a level of concurrency control that is vital for correct data handling in concurrent programming.

In summary, the designation of a 'volatile' variable effectively signals to the program that the variable can change unexpectedly, thereby prompting necessary precautions around its access and modification.