Floating Point Number + NAN

[sonia.sardana]

#include<stdio.h>
#include<conio.h>
float square(float);
void main()
{
    clrscr(); 
    float a,b;
    printf("ENter a Number");
    scanf("%f",&a);
    printf("Number=%f",a);
    b=square(a);
    printf("\nSquare of %f is %f",a,b);
   getch();
}

float square(float x)
{
    float y;
    y=x*x;
    return(y);
}

In the above program, I am calculating the square of float number. But sometimes the number is entered as NAN and sometimes Output is NAN. What is NAN? I am entering floating point number, then y NAN is entered?
SEE the Image attached for the OUTPUT.