from
The Free On-line Dictionary of Computing (8 July 2008)
volatile variable
<programming> 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 {interrupts}) 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)