7 (seven) is a positive integer following 6 and preceding 8. Its ordinal form is written "7th" or "seventh".
7 is an odd prime number. It is a Mersenne prime, since it is equal to 2^3-1.
It is associated with good luck in Western culture.
It is the third number in Sylvester's sequence.
Some googologisms based on 7 are googolseptiplex, supersept and trisept.
In Greek-based number naming systems, 7 is associated with prefix hepta-, and with prefix septi- in Latin systems.
Using the gag prefix, seven can be called gagtwo, since A(2,2) = 7.
7 is the last digit of Graham's number, and in fact it is the last digit of any power tower of 3's where the height of the tower is at least 2.