Seventeen (17) is a positive integer one more than 16 and one less than 18.
17 is the seventh prime number, after 13. It is in a twin prime pair with 19.
17 is also a Fermat prime, between 5 and 257. As such, a regular polygon with 17 sides is constructible with a compass and straightedge.
17 is a Mersenne exponent producing 131071.