league 普通用词，指为了共同的利益而建立的地区性、全国性或国际性的联合组织，也指民间或半官方组织的联盟。
federation 泛指用条款固定下来的一切联合，尤指联邦政府。
union 普通用词，指由若干个体结成的一个统一的整体。强调其永久性。
alliance 普通用词，指对双方均有利的任何联盟。
参考例句
1. Future report released by the World Federation of UN Associations says the future is bright. 「联合国同志会世界协会」发表「未来展望报告」，并表示未来是光明的。
2. Most people have the wrong impression about the World Wrestling Federation. We're a nonviolent form of entertainment. -We never use sex to enhance our image. 大多数人都对世界摔跤联合会有着错误的印象。我们是非暴力形式的娱乐。-我们从不用性来增强我们的形象。
3. Despite all this consolidation, the World Federation of Exchanges still counts 54 bourses among its members, which between them account for 97% of global stock trading. 尽管有这些合并，世界交易所联合会的会员仍然有54个交易所，而这些交易所占了世界股票交易的97%。
英英解释
n.
1. an organization formed by merging several groups or parties
2. a union of political organizations
3. the act of constituting a political unity out of a number of separate states or colonies or province
