The Hebrew word acher means 'another' or 'other' — that which is different, subsequent, or separate from what has been mentioned. It is used throughout the Old Testament to distinguish between things, people, or gods, and carries particular theological weight in texts about idolatry and covenant exclusivity.
Acher is theologically loaded wherever God commands Israel to have 'no other gods' before Him (Exodus 20:3). The word 'other' in that commandment is acher — these are foreign, alien deities that are utterly distinct from YHWH. The concept of the 'other' drives much of Israel's covenantal struggle: the temptation to follow other gods, make other alliances, and trust in other sources of security. The prophets use acher as a term of alarm — a warning that Israel has turned to what is alien and false. In positive usage, it points to the promise of 'a new heart' and 'another spirit' given by God.