Annulus

The annulus (or annular space) of an oil well or water well is any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept. The presence of an annulus gives the ability to circulate fluid in the well, provided that excess drill cuttings have not accumulated in the annulus, preventing fluid movement and possibly sticking the pipe in the borehole.
Annulus is the space between:

1. the casing and the wall of the borehole.
2. two strings of casing.
3. tubing and casing.