A collection of circles is said to be concentric if they have the same center. The region formed between two concentric circles is therefore an annulus.
In the following picture, the gray region is the annulus delimited by the two black concentric circles.
{0.7} \psdots(0,0) \end{pspicture}](http://images.planetmath.org:8080/cache/objects/3277/js/img1.png "\begin{pspicture}(-2,-2)(2,2) \psset{linewidth=0.5mm} \pscircle[fillstyle=solid,... ...pscircle[fillstyle=solid,fillcolor=white](0,0){0.7} \psdots(0,0) \end{pspicture}")