In general, allies are people or groups that have joined an alliance and are working together to achieve some common purpose. In general English usage, those who share a common goal and whose work toward that goal is complementary may be viewed as allies for various purposes even when no explicit agreement has been worked out between them. Similarly, when the term is used in the context of war or armed struggle, a formal military alliance is not required for being perceived as an ally — co-belligerence, to fight alongside someone, is enough. According to this general usage, allies become allies not when concluding an alliance treaty but when struck by war.
In the context of diversity politics, an ally has been defined as "a person of one social identity group who stands up in support of members of another group; typically a member of dominant group standing beside member(s) of a group being discriminated against or treated unjustly; e.g., a male arguing for equal pay for women." (This definition is adapted from one developed by the Arizona State University Intergroup Relations Center).
Yet another meaning of allies is found in the books of Carlos Castaneda, describing a race of non-human but human-appearing beings which inhabit the earth, and only infrequently interfere with human endeavors. Similar beings exist in various other fictional (and possible non-fiction) works including the book The Holy by author Daniel Quinn, in which one character refers to these beings as "you-whos". These beings may also be related or identical to descriptions of demons or nephilim.
The term is generally used in the generic sense of "all who opposed the enemy". In addition, it is usually used in a strict dichotomy of them vs. us, reflecting wartime propaganda, with no account taken of nuances of countries that were occupied as neutrals, changed sides or participated in concurrent wars.
In previous major European wars, e.g., those against the declarers of war Louis XIV of France, Louis XV of France, and Napoleon, the term coalition was used because these were not considered total wars, and the sovereign nations could enter and leave belligerency with diplomatic agreements with the enemy.af:Geallieerdes cs:Spojenci de:Alliierte fa:نیروهای متفقین fr:Alliés ga:Comhghuaillithe ko:연합국 it:Alleati he:בעלות הברית nl:Geallieerden ja:連合国 pl:Alianci sl:Zavezniki zh:同盟国