quadrant

The $x$- and $y$-axes the $xy$-plane in four right-angle domains which are called the quadrants of the plane.  They are numbered going round the origin in anticlockwise direction so that

$$\{(x,y)\mid x>0,y>0\}$$

is the first quadrant,

the second quadrant, and so on.

Naturally, one can speak of the quadrants of the complex plane, too.

The lines $y=\pm x$ have as their slope angles $\pm {45}^{\circ }$,thus halving the quadrant angles; they are called thequadrant bisectors.  Cf. angle bisector as locus.