If $(u, v)\in W^{1, p}(\Omega)\times W^{1, q}(\Omega)$, what does the notation $|(u, v)|$ mean?

Question

Let $\Omega$ be an open bounded domain, $p, q>1$ and let $W= W^{1, p}(\Omega)\times W^{1, q}(\Omega)$. Let $(u, v)\in W$.

What does the notation $|(u, v)|$ mean?

It is in my math class notes, but. I don't understand it. Could someone please help?

Thank you in advance!