A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 854658 can be evenly divided by 1 2 3 6 7 9 14 17 18 19 21 27 34 38 42 49 51 54 57 63 98 102 114 119 126 133 147 153 171 189 238 266 294 306 323 342 357 378 399 441 459 513 646 714 798 833 882 918 931 969 1,026 1,071 1,197 1,323 1,666 1,862 1,938 2,142 2,261 2,394 2,499 2,646 2,793 2,907 3,213 3,591 4,522 4,998 5,586 5,814 6,426 6,783 7,182 7,497 8,379 8,721 13,566 14,994 15,827 16,758 17,442 20,349 22,491 25,137 31,654 40,698 44,982 47,481 50,274 61,047 94,962 122,094 142,443 284,886 427,329 and 854,658, with no remainder.
Since 854,658 cannot be divided by just 1 and 854,658, it is not a prime number.