Shadowing occurs when a name is redefined in a place where there is already an active definition of that name. For example, x might be redefined at a place where x is in scope. It can also happen where you define local variable insert where there is already something called insert defined outside.

Java disallows shadowing. Here is an example from C.

  int sumsq(int n)
  {
    int sum = 0;
    for(int i = 0; i < n; i++)
    {
      int n = i*i;
      sum  += n;
    }
    return sum;
  }