What does it mean denumerable sequence of sets? I've been searching for the definition of $\textbf{denumerable sequence of sets}$, but I have not found it yet. 

Can someone tell me what doest it mean a denumerable sequence of sets? Thank you!
If for each $n \in \mathbb N$ a set $M_n$ is given, then you have the sequence

$$(M_1,M_2,M_3,....).$$

Such a sequence is a denumerable sequence of sets.
If you can label each element in the set with a real number, it is denumerable. If a set is denumerable, it is countable. This is a way, in which we may have an infinite countable set