Domini Minores, literally “lesser lords” in Old Valyrian, is a catchall class for all those who have the dignity of being referred to as “lord” or “lady” in Valyria but are neither dukes nor counts. This includes feudal lords whose tenure does not approach the size of that of a peer, but also those who are lords by several other means, including simply by courtesy.
Generally, domini minores as a Valyrian class corresponds with the “aristocracy” tier in Chronicles of Elyria, which is associated with barons and lords mayor. While in the historical real world, there was no class that fell between nobility and gentry, that Chronicles of Elyria includes one is no problem for Valyria, as the kingdom simply aligns this category with the lower ranks of its feudal lords and the upper ranks of its gentry. Thus, in Valyria barons are not peers, but merely minor lords, and so high powered knights like bannerets are counted as lords as well.
Finally, there are many other people who hold no feudal title at all, whether as a peer or as gentry, whom Valyria still regards as “lords.” This custom is called awarding titles by courtesy, and is afforded to mayors, cabinet officers, and the close family of magnates.