a captured population seen as one body: the prisoners an army carries off, and the captivity they are carried into
This word belongs to the vocabulary of war. When one army beat another, it came home with two kinds of takings: goods, and people. This word names the second kind, and it names them as a single mass rather than one prisoner at a time. In 2 Chronicles 28:5 the army of Aram carries off "a great" body of captives, with the word "great" describing the whole crowd as though it were one thing, and in 2 Chronicles 28:14 the fighting men set down the captives beside the plunder in front of the officials.
From there the word moves easily to the condition itself. A group of captives is defined by what has happened to them, so the same word can name the state instead of the people: Nehemiah prays that his enemies be handed over as plunder in a land of captivity, and Jeremiah says Moab's daughters have gone into captivity. Which of the two a verse means is settled by the sentence around it, not by the word.
Most of its uses sit inside one story. Five of its nine appearances fall in 2 Chronicles 28, where a northern army drags Judean prisoners home and a prophet orders them fed, clothed, and sent back. So this is not a technical name for the Babylonian exile, the way the standard word for that deportation (גּוֹלָה) is; it is the plain battlefield word for the people a victory produces. Its close partner, the more common noun for captives and captivity (שְׁבִי), covers much of the same ground, and Jeremiah 48:46 sets the two side by side in a single line, one for Moab's sons and the other for its daughters.