1. A close association of nations, people, or other groups who agree to cooperate, and which are formed to advance common interests or causes.
2. A connection based on kinship, marriage, or common interest; a bond or tie.
3. A similarity in nature or type; an affinity.
4. The act of becoming allied or the condition of being allied.
5. Etymology: from Old French aliance, from alier, "to combine, to unite".

Originally it referred to "a union by marriage".