7

Seven (7) is a positive integer one more than 6 and one less than 8.

Properties
7 is the fourth prime number (between 5 and 11). It is a Mersenne prime exponent (producing 127), and is itself a Mersenne prime (between 3 and 31).

7 is the fourth Lucas number, coming between 4 and 11.

7 is in a twin prime pair with 5.

7 is widely considered a lucky number. It is also a lucky number in the numerical sense (between 3 and 9).

Seven is the smallest number of sides of a regular polygon that cannot be constructed with a compass and straightedge alone.