A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 2506608 can be evenly divided by 1 2 3 4 6 8 9 12 13 16 18 24 26 36 39 48 52 72 78 103 104 117 144 156 169 206 208 234 309 312 338 412 468 507 618 624 676 824 927 936 1,014 1,236 1,339 1,352 1,521 1,648 1,854 1,872 2,028 2,472 2,678 2,704 3,042 3,708 4,017 4,056 4,944 5,356 6,084 7,416 8,034 8,112 10,712 12,051 12,168 14,832 16,068 17,407 21,424 24,102 24,336 32,136 34,814 48,204 52,221 64,272 69,628 96,408 104,442 139,256 156,663 192,816 208,884 278,512 313,326 417,768 626,652 835,536 1,253,304 and 2,506,608, with no remainder.
Since 2,506,608 cannot be divided by just 1 and 2,506,608, it is not a prime number.