What does this domain notation "\" mean?

Question

I have to compute an integral with the domain $D$. I don't know what the "\" means. Here is the domain description. D = {(x, y) : x² + y² $\leq 4$} \ {(x, y) : (x - 1)² + y² $\leq 1$}

Answer 1

It means 'but not' So it would refer do the region being a circle centered at the origin but excluding the region (x-1)^2 + y^2 <= 1

Answer 2

$D_1 := \{(x,y) ~:~x^2+y^2\le4\}$ is the interior of a disk centered at the origin with radius $2$.

$D_2 := \{(x,y) ~:~(x-1)^2+y^2\le1\}$ is the interior of a disk centered at $(1,0)$ with radius $1$. $D_2$ lies entirely inside $D_1$ except for a single intersection at $(2,0)$.

In this context, $\setminus$ denotes the set difference operation, so $D=D_1 \setminus D_2$ is the set of points belonging to the first disk, but not to the second disk. This is the crescent-shaped region shown in Wolfram|Alpha's "inequality plot".