Disk

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Disk[{$c_x$, $c_y$}, r]

fills a circle with center ($c_x$, $c_y$) and radius r.

Disk[{$c_x$, $c_y$}, {$r_x$, $r_y$}]

fills an ellipse.

Disk[{$c_x$, $c_y$}]

chooses radius 1.

Disk[]

chooses center $(0, 0)$' and radius 1.

Disk[{x, y}, ..., {$t_1$, $t_2$}]

is a sector from angle $t_1$ to $t_2$.

The outer border can be drawn using EdgeForm:

Disk can also draw sectors of circles and ellipses

Directive
EdgeForm