The coordinate planes ($yz$ -plane, $zx$ -plane, $xy$ -plane) divide 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. One calls the octant, where all three coordinates are nonnegative, the first octant. The other octants have no agreed numbering; see still Wikipedia.