A more thorough explanation:

Definition: Domina (dom-uh-nuh), n. [Law Latin] 1. A lady. 2. A peeress. Compare with Dame(1), (2).

• Queen Elizabeth II is a domina as she is a peeress.
• The term "domina" was commonly used in ancient Rome to refer to a lady of high social status.

The examples illustrate that a domina is a woman of high social status, either as a peeress or a lady. Queen Elizabeth II is an example of a domina as she is a peeress, while the second example shows that the term has been used historically to refer to women of high social standing.