a group or set of four persons or things.

Bookbinding . four gathered sheets folded in two for binding together.

Mathematics .

an expression of the form a + bi + cj + dk, where a, b, c, and d are real numbers; i 2 = j 2 = k 2 = ?1; and ij = ?ji = k, jk = ?kj = i, and ki = ?ik = j.

a quantity or operator expressed as the sum of a real number and three complex numbers, equivalent to the quotient of two vectors. The field of quaternions is not commutative under multiplication.