al·li·ance 
(ə-līəns)
(ə-līəns)n.
1.
a. A close association of nations or other groups, formed to advance common interests or causes: an alliance of labor unions opposing the bill.
b. A formal agreement establishing such an association, especially an international treaty of friendship.
2.
a. A connection based on kinship, marriage, or common interest; a bond or tie: the shifting alliances within a large family.
b. A long-term association of two or more animals of the same species, in which members cooperate for a purpose such as gaining access to mates or defending territory.
3. Close similarity in nature or type; affinity: the ancient alliance between mathematics and music.
4. The act of becoming allied or the condition of being allied: the church, acting in alliance with community groups.
[Middle English, from Old French aliance, from alier, to ally; see ALLY.]