Source : Free On-Line Dictionary of Computing

volatile variable
     
         A variable in a computer program which can be
        modified by processes other than the program.  For example, a
        variable that stores the value of a timer chip (either because
        it is located at the address of the hardware device or because
        it is updated on {interrupt}s) needs to be volatile to be
        useful.
     
        A {static} variable, on the other hand, maintains its value
        until the program changes it or it is no longer needed.  In
        addition, volatile variables can be held in the {stack}
        whereas static variables are usually stored in a program's
        {data segment}.
     
        (1995-05-17)