Static

The static keyword is used to create variables that are visible to only one function. However unlike local variables that get created and destroyed every time a function is called, static variables persist beyond the function call, preserving their data between function calls.

Variables declared as static will only be created and initialized the first time a function is called.

 
/* RandomWalk
* Paul Badger 2007
* RandomWalk wanders up and down randomly between two
* endpoints. The maximum move in one loop is governed by
* the parameter "stepsize".
* A static variable is moved up and down a random amount.
* This technique is also known as "pink noise" and "drunken walk".
*/
 
#define randomWalkLowRange -20
#define randomWalkHighRange 20
int stepsize;
 
int thisTime;
int total;
 
void setup()
{
  Serial.begin(9600);
}
 
void loop()
{        //  tetst randomWalk function
  stepsize = 5;
  thisTime = randomWalk(stepsize);
  Serial.println(thisTime);
   delay(10);
}
 
int randomWalk(int moveSize){
  static int  place;     // variable to store value in random walk - declared static so that it stores
                         // values in between function calls, but no other functions can change its value
 
  place = place + (random(-moveSize, moveSize + 1));
 
  if (place < randomWalkLowRange){                    // check lower and upper limits
    place = place + (randomWalkLowRange - place);     // reflect number back in positive direction
  }
  else if(place > randomWalkHighRange){
    place = place - (place - randomWalkHighRange);     // reflect number back in negative direction
  }
 
  return place;
}

Source: arduino.cc