Full Idea
X1 x X2 x X3... x Xn indicates the 'cartesian product' of those sets, the set of all the n-tuples with its first member in X1, its second in X2, and so on.
Gist of Idea
X1 x X2 x X3... x Xn indicates the 'cartesian product' of those sets
Source
Graham Priest (Intro to Non-Classical Logic (1st ed) [2001], 0.1.0)
Book Reference
Priest,Graham: 'Introduction to Non-Classical Logic' [CUP 2001], p.-5