Let R be a relation from a set A to a set B. Then the of all first components or coordinates of the ordered pairs belonging to R is called the domain of R.
Clearly, domain of R ⊆ A
If A = {1, 3, 5, 7}, B = {2, 4, 6, 10} and R = {(1, 8), (3, 6), (5, 2), (1, 4)} is a relation from A to B, then
Domain( R) = {1, 3, 5}