I. noun 1. a union or association formed for mutual benefit, especially between countries or organizations • a defensive alliance between Australia and New Zealand
• divisions within the alliance.
2. a relationship based on similarity of interests, nature, or qualities • an alliance between medicine and morality.
3. [ mass noun] — the state of being joined or associated • his party is in alliance with the Greens.
[Ecology] a group of closely related plant associations.
– origin Middle English: from Old French aliance, from alier ‘to ally’ (see ally1).