authoritative instruction: binding teaching given by a recognized authority, and in time the whole body of God's teaching gathered into a written book
At its heart this word means instruction that carries weight because of who gives it. The verb behind it pictures a hand stretched out to point the way, so the noun is the pointing itself: a directive you are meant to follow. Who does the pointing varies, and that variety is what spreads the word across so many settings. A priest hands down a ruling on what is clean or holy. A parent teaches a child how to live. A prophet delivers God's word to the people. Moses lays a whole covenant before Israel. Later the word names the written book that holds all of it.
These are not different words but one word stretching to cover a growing thing. The earliest uses are concrete and plural: many separate instructions on many separate matters. Over time, and decisively in Deuteronomy, the separate instructions are gathered into a single written document set before the whole nation, and the word shifts toward what English calls a law book. By the time of Ezra it can mean the five books of Moses read aloud to the assembly, and in the Psalms it becomes something a person loves and turns over in the mind day and night.
The one thread through all of it is authority joined to teaching. It is never bare information and never mere rules cut off from a teacher and a relationship. That is why flattening it into the modern idea of law, in the sense of legislation passed and enforced, loses what the word is really about. It is instruction you can trust because of its source, whether that source is a priest at the sanctuary, a mother at home, a prophet in the street, or the God of the covenant in a written book.