What does this "$\times$" in $(\Bbb Z/8\Bbb Z)^\times$ mean?

Question

I saw this $\times$ symbol several times, but couldn't find what it means.

Answer 1

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$(\mathbb Z/8\mathbb Z)^\times$ means the multiplicative group of units (invertible elements) modulo $8$.

It's $\{1,3,5,7\}$ with multiplication mod $8$ as a group.

Answer 2

$(\Bbb Z/8\Bbb Z)^\times$ is also the set of integers $n$ such that

$$1\le n <8 \text{ and } \gcd(8,n)=1.$$

It contains $\phi(8) $ elements, where $\phi$ is the Euler totient function.