a woman of the ruling class: a princess, queen, or noble lady, whose rank comes from the household she belongs to
This is the feminine of the ordinary word for a chief or official (שַׂר), and that is the key to its range. It does not name one fixed title. It names a woman who stands inside the ruling class, and how high she stands depends on whose household she belongs to.
So the same word reaches from a commander's ladies-in-waiting up to the royal wives of foreign kings. When the writers of Kings need to set Solomon's wives of royal birth apart from his concubines (פִּלֶגֶשׁ), this is the word they use. When Isaiah wants to shame the great powers by making their royal women nurse Judah's children, this is the word he sets beside "kings." Hebrew had plainer options available: one word for the wife of a reigning king (מַלְכָּה) and another for the powerful mother or mistress of the palace (גְּבִירָה). This word was chosen when rank itself, rather than a particular office, was the point.
It appears only five times, and never casually. Each use is deliberate, and the last is the boldest: Lamentations calls the ruined city a great lady among the provinces, playing on the ordinary word for an imperial district. A city that once collected tribute is now doing forced labor, and the word carries the whole reversal in one title.