I am having a hard time understanding the meaning of the union operation in this equation.
$C(A)=\bigcup_{x \in A}C(x)$
For context, here is the sentence:
The candidate set for $x$ is $S \cap C(x)$. The candidate region for a set of points $A$ is $C(A)=\bigcup_{x \in A}C(x)$, with the candidate set $S \cap C(A)$. [1]
Is this an indexed union? Also, I haven't seen a union operation except for when it appears between two sets.
Any advice would be greatly appreciated. Thanks.
[1] Clarkson, 1988. "A randomized algorithm for Closest-Point Queries".