Although your mention of feudal Japan is unspecific (since that could refer to the Edo period, or the Sengoku period, or the medieval period, or simply the entire period of warrior government), I can say one thing:

Daimyō in the sense of a title, like duke or count would be, didn’t exist before the Edo period: although the word itself is much older (we can find it in 11th century sources), it took on various meanings over time.

In the Edo period, it referred to a direct vassal to the shogun with a domain income of 10.000 koku or more.

In historical research, we use the terms sengoku daimyō and shugo daimyō to refer to powerful warriors in the Sengoku and Muromachi periods, respectively—but these are research terms, not terms people back then actually used.

Historical sources in the medieval period, from the 12th to the 16th century, use the word in reference to warriors who wield great influence, who were commanding a large number of followers or controlling large swaths of land. This meaning is derived from another word that is written with the same Chinese characters, 大名, pronounced taimei, which means “great fame, reputation, or distinction.” We can find this usage in sources from the 13th century onward.

And before that, as early as the 11th century, daimyō was used to refer to someone who possessed a lot of myōden—"named fields,” which formed the basic unit of land taxation. It could also literally be used to refer to very large fields of this kind.

I do think it is quite clear that this change of usage over time does follow a rather obvious.

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So, since we established that the word had a very ambiguous use before the Edo period, how was it during the Edo period? A database search over at the Historiographical Institute of the University of Tokyo's Kinseishi hensan shien deetabeesu offers a mere 199 results for the term daimyō, but almost exclusively in referral to “the daimyo (plural) [did something, etc.],” not when actually referring to individuals. (granted I only glanced over the results, and the sources recorded in here are only a fraction of what's out there in total)

But there are various other ways of referring to, or addressing people. The most important one remained fairly consistent through all of premodern history: referring to their title or office within the imperial court's hierarchy.

Powerful warriors (daimyo) during the Edo period were still appointed to these old court offices, especially provincial governorships, officers in the imperial and palace guards, or stablemasters in the imperial palace, and so on. Of course, these titles had lost their actual meaning centuries ago, but remained in wide use in the form of honorary titles. You would typically speak of, say, the “Mutsu no kami [governor of Mutsu (province)]” when talking about the daimyo of the Date family, who ruled over Sendai-han, and were traditionally appointed to that position. (the same database as consulted above gives about 2.500 hits for this type of naming pattern)

In the same way, daimyo would sign their official documents with these court titles.

When talking about or addressing such a person, you might find something along the lines of Mutsu no kami-dono or Mutsu no kami-sama (both -dono and -sama are suffixes to imply respectfulness, although -dono is an older word). A common variant is having the family name precede the title, in the form of "Date Mutsu no kami."

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[corrections might be incoming via u/ParallelPain so no guarantees for perfect accuracy until he saw this - he should have seen significantly more sources from the Edo period than I have *laughs*)