210

210 = 2 x 3 x 5 x 7 (the first four primes).

210 is both a triangular number and a pentagonal number. It is the smallest such number (other than 1); the next smallest is 40,755.

210 is the smallest area of two different primitive Pythagorean triangles: 20, 21, 29 and 12, 35, 37.

The primes 47, 257, 467, 677, 887, 1097, and 1307 constitute a progression of seven terms with a common difference of 210.

210 Isabella is a large, dark main-belt asteroid discovered in 1879.

Source: The Penguin Dictionary of Curious and Interesting Numbers, rev. ed., by David Wells.