Why is A union B also called "A or B"? In A union B, the element either belongs to A or B, or A and B right?
So shouldn't it be called A and/or B? Due to this I am unable to solve a problem in my textbook.

Because the elements of $A\cup B$ are exactly those objects that either belong to $A$, or belong to $B$. The claim "$p$ or $q$" is true if either $p$ is true, or $q$ is true, or $p$ and $q$ are both true.
