octant

The coordinate planes ( $y z$ -plane, $z x$ -plane, $x y$ -plane) the space $\mathbb{R}^{3}$ into eight trihedral angles, which are called the octants of the space. The magnitude of each octant as a solid angle is $\frac{\pi}{2}$ .

Any coordinate of a point moving in a certain octant preserves its sign (http://planetmath.org/SignumFunction). One calls the octant, where all three coordinates are nonnegative, the first octant. The other octants have no agreed numbering; see still http://de.wikipedia.org/wiki/Oktant_(Geometrie)Wikipedia.