volatile variables :

Volatile variables are same like other variables but there are values might be changed at any given point only by some external resources. 

syntax: 

volatile int number;

volatile variables are qualifiers that show us that value of the variables can be change at any time in the code source.The value may be discard by some external factor, if it does't appear to the left  of the given statement.volatile variables stored in the compiler.  

example:- 

volatile int n;