Pronic
Neighbors multiplied — born a pair.
A product of two consecutive integers n(n+1), like 6 = 2×3, 42 = 6×7 or 990 = 99×100. Also called a pronic or oblong number.
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The range holds exactly 1,000 pronic numbers (0×1 through 999×1000), or 0.1% — almost exactly as rare as squares. No coincidence: n(n+1) differs from n² by just n, and the two dance around the same parabola.
Every pronic number is even (one of two consecutive integers must be), and each is exactly twice a triangular number: n(n+1) = 2×Tₙ. So a pronic number is a doubled triangle — geometrically, a rectangle of dots one row short of a square.
Trivia: 42 = 6×7 is the Answer to Life, the Universe and Everything in The Hitchhiker's Guide — and it's a pronic number. At the other extreme, 2 = 1×2 is the only prime pronic number, since every larger one carries an even factor.