a writing: text set down in written form, especially an official record, list, or decree.
This word is the noun "a writing," the thing produced when someone writes. It belongs almost entirely to the later books of the Old Testament, where written records run the machinery of empire: the census lists of returning exiles, the sealed orders of the Persian court, the archives of kings. Wherever it appears, the idea is the same, something fixed in written form.
From that basic idea the word fans out in a few directions. A written list of names becomes an official register. A king's order, once written and sealed, becomes an edict. The very characters a people write in become their script. And a plain document or set of directions is simply a writing. The book of Esther shows several of these at once: each province receives the royal order "according to its own script and its own language," so the written decree and the writing system stand side by side in a single verse.
It helps to set this word beside its neighbors. A scroll or bound book, the physical object you hold, is a different word (סֵפֶר). A single piece of writing or inscription is מִכְתָּב. This word leans instead toward the written text itself and the official act of putting something on record.