The meaning of notation $\subset\subset$ in complex analysis

complex-analysis notation

I have read the book Function Theory of Several Complex Variables of Krantz. But there is a notation the meaning of which I don't know. The notation is $\subset\subset$. For example, "let $\Omega\subset\subset \mathbb{R}^{n}$ be a connected open set".

Can somebody give me a definition?


Solution 1:

There's an index of notation at the back of the book. Apparently, $\subset \subset$ means ‘relatively compact in’ and is defined in section 1.1.

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