An adversary or accuser — one who opposes. Used of human opponents, of the angel blocking Balaam's path, and of the cosmic Accuser in Job and Zechariah. The NT personalizes this as the Devil.
In Job 1-2 and Zechariah 3, ha-satan (the Accuser) functions in a heavenly court. The definite article suggests a role, not yet a personal name. By the NT, Satan is clearly a personal, fallen being opposing God.
Zechariah 3 gives the clearest OT picture: Satan accuses; God rebukes Satan and clothes Joshua in clean garments — a preview of the gospel. Jesus' declaration 'I saw Satan fall like lightning' (Luke 10:18) shows the decisive defeat of the Accuser.