The Free On-line Dictionary of Computing (30 December 2018):

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 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)