An alliance is an agreement made between two or more parties to benefit them.
The word is used mainly in war. Two or more countries that make an alliance promise to help one other during war. Such countries are called allies. Those countries are said to be allied.
An example of an alliance with more than two countries is the Central Powers during World War I. That alliance was made up of Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria.
The English Wiktionary has a dictionary definition (meanings of a word) for: alliance