A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 206640 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 24 28 30 35 36 40 41 42 45 48 56 60 63 70 72 80 82 84 90 105 112 120 123 126 140 144 164 168 180 205 210 240 246 252 280 287 315 328 336 360 369 410 420 492 504 560 574 615 630 656 720 738 820 840 861 984 1,008 1,148 1,230 1,260 1,435 1,476 1,640 1,680 1,722 1,845 1,968 2,296 2,460 2,520 2,583 2,870 2,952 3,280 3,444 3,690 4,305 4,592 4,920 5,040 5,166 5,740 5,904 6,888 7,380 8,610 9,840 10,332 11,480 12,915 13,776 14,760 17,220 20,664 22,960 25,830 29,520 34,440 41,328 51,660 68,880 103,320 and 206,640, with no remainder.
Since 206,640 cannot be divided by just 1 and 206,640, it is not a prime number.