Actually another Title borrowed from Ancient Greece (which associated the word "Tyrannos"), the position of Dominus (and it's feminine "Domina") - were seen in usage in the 2nd Century AD. Originally not a "legal" Title, it was afforded such status under Emperor Diocletian. By strict definition, "Dominus" means to "dominate or own". Seen as a Slave Owner and Seller, The Dominus was a form of "Lesser" Nobility. Often looked down upon by "Upper" Class Romans, such interactions were normally accomplished through intermediaries. Still, a Dominus was known for his quality of Slaves maintained and sold. Most consignments were destined as Household Servants. Some Dominus and Domina specialized in the "Fighting" Class (or Gladiators).
Roman women were traditionally not Business Owners, with certain exceptions. A Mobile Woman was expected to conduct her affairs through Senior Household Servants. A Domina was one such position were such protocols were not enforced. This made a sometimes uncomfortable "interaction" between a "Businessman" and a "Domina". Later, the Title would evolve in both Italy and Spain into that of "Dom" or Sir. It was also associated with a Landowner.