Chapter 9 - Polar Coordinates; Vectors - 9.1 Polar Coordinates - 9.1 Assess Your Understanding - Page 571: 5

pole; polar axis

The pole is basically the center of the polar coordinate system so it coincides with the origin of the rectangular coordinate system, whereas the polar axis is to the right of the pole so in the rectangular coordinate system, it coincides with the positive $x$-axis. Hence, the missing expressions in the given statement, in order, are: pole and polar axis