7

Seven is the natural number that follows six and precedes eight. In decimal notation it is a single-digit number. It is the fourth prime number, after 2, 3, and 5, and it is odd. In number theory, 7 is a Mersenne prime since 7 = 2^3 − 1, with an exponent that is prime; it is also a safe prime because (7 − 1)/2 = 3 is prime. The factorial of seven is 7! = 5040.

In science, the atomic number 7 identifies nitrogen, a nonmetal essential to life and making up about 78% of Earth's atmosphere by volume. The seventh planet from the Sun is Uranus. In numeral systems beyond decimal, base-7 uses digits 0 through 6.

Culture and timekeeping associate seven with various symbols and traditions. July, named after Julius Caesar, is commonly the seventh month in the modern calendar. The week is typically divided into seven days. A rainbow is often described as having seven colors. Many cultures refer to seven wonders of the ancient world, seven continents in popular models, and seven motifs in folklore and religion.

In geometry and games, a regular heptagon has seven sides and presents geometric challenges for exact construction with a straightedge and compass. In probability, rolling two six-sided dice yields 7 as the most likely sum, with six distinct dice combinations producing it. The number seven also appears in a wide range of mathematical, scientific, and cultural contexts, contributing to its prominence in systems of counting and symbolism.