al‧li‧ance W3 [countable]
an arrangement in which two or more countries, groups etc agree to work together to try to change or achieve something
make/enter into/form/forge an alliance (=agree to work together)
The companies have formed an alliance to market the product.
a group of two or more countries, groups etc who work together to achieve something:
independent organizations and alliances
if two groups, countries etc are in alliance, they work together to achieve something or protect each another:
Relief workers in alliance with local charities are trying to help the victims.
a close relationship, especially a marriage, between people