Explain the Defination and Declaration of Union?

Generally in terms the composition of a union may be defined as be as

union tag
{
member 1;
member 2;
member m;
};

Where union is a necessary keyword and the other terms have the same meaning as in a structure definition.

Individual union variables are able to then be declared as

storage-class union tag variable1, variable 2, ..., variable n;

Where storage-class is an elective storage class specifier, union is a needed keyword, tag is the name that appeared in the union definition and variable 1, variable 2, ..., variable n are union variables of type tag.