问一个方程中的小问题

#include <stdio.h>
#include <math.h>

void main()
{
    float a,b,c,disc,x1,x2,realpart,imagpart;
    scanf("%f,%f,%f,%f",&a,&b,&c);
    printf("The equation");
    if(fabs(a) <= 1e-6)
        printf("is not a quadratic\n");
    else
    {
        disc = b*b - 4*a*c;
        if(fabs(disc) <= 1e-6)
        {
            printf("has two equal roots: %8.4f\n", -b/(2*a));

        }
        else
          if(disc > 1e-6)
          {
            x1 = (-b + sqrt(disc))/(2*a);
            x2 = (-b - sqrt(disc))/(2*a);
            printf("has distinct real roots: %8.4f and %8.4f\n",x1,x2);
          }
        else
        {
            realpart = -b/(2*a);
            imagpart = sqrt(-disc)/(2*a);
            printf("has complex roots: \n");
            printf("%8.4f + %8.4fi\n",realpart,imagpart);
            printf("%8.4f - %8.4fi\n",realpart,imagpart);
        }
            
    }
}

这个方程中的“1e-6”是什么意思啊？？？在数学中怎么表示的？？？