In a relational model, each attribute carries a domain of its own.
A domain refers to the set of unique values used to define that attribute and will act as a "model" set of values.
These values, being unique to that attribute are referred to as "atomic values".
Thus, in layman terms, a domain is a set of acceptable values that a column is allowed to store.
(i) Let's suppose you have an attribute called "Marital_Status". The domain of this attribute has a set of possible unique (or atomic or distinct or elemental) values: 'MARRIED','SINGLE', 'DIVORCED';