Prime
No divisors, no compromises.
A number greater than 1 divisible only by 1 and itself, like 2, 73 or 999983. The fundamental theorem of arithmetic says every integer is built from primes in exactly one way — they are the atoms of the integers.
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The prime number theorem says the density of primes near n is about 1/ln n, thinning forever. Across the full 0–1,000,000 range there are 78,498 primes, about 7.85%; among six-digit numbers only 68,906 remain, 7.7% — the bigger a number gets, the harder it is to stay unfactorable.
The primes never run out: Euclid proved it over two thousand years ago. Multiply all the primes together, add 1, and the result is either prime itself or hides a prime factor missing from your list — contradiction. It remains the textbook example of beauty through brevity.
Trivia: 2 is the only even prime and the only number that is simultaneously prime, factorial, Fibonacci and a power of two. The largest prime in range is 999983, and the largest twin-prime pair is 999959 and 999961 — one short hop from a million.