What is alliance? What is alliance? A close association of nations or other groups, formed to advance common interests or causes: an alliance of labor unions opposing the bill. A formal agreement establishing such an association, especially an international treaty of friendship.A connection based on kinship, marriage, or common interest; a bond or tie: the shifting alliances within a large family. Close similarity in nature or type; affinity: the ancient alliance between mathematics and music. The act of becoming allied or the condition of being allied: the church, acting in alliance with community groups.