Answer:
Domain is the set of all possible input for which the relation is defined.
Explanation:
If a relation is defined as

In this relation x is the variable that defines all inputs of the relation and y is the variable that defines all outputs of the relation.
Domain is the set of all possible input for which the relation is defined. So, the domain of the above relation is

In the ordered pairs, the set of x-coordinates is called domain of the relation.