外来种指原来在当地没有自然分布，经由人为无意或有意引进的物种。而已在自然环境创建稳定族群者，则称为「归化种」。外来种的好坏由于人类社会的定义而有所不同：并非所有的外来种都有害，其实大部份都对人类有益，才会被人为引进，如加州蜜李、美国樱桃等。会破坏生态环境及造成人类经济损失的外来种则称为「入侵种」，如红火蚁、福寿螺、布袋莲、非洲大蜗牛、巴西龟、松材线虫等。

由于人类在世界各地交流频繁，使得许多生物得以突破地理隔绝，拓展至他处。外来种移入后，可能无法适应新的栖息地而很快绝迹，也有可能适应新环境；若新环境没有天敌的控制，加上繁殖力旺盛，那么外来种就会变成入侵者，很快会与原生物种发生竞争，破坏当地生态平衡，甚至造成对人类经济的危害。

An introduced, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its native distributional range, which has arrived there by human activity, either deliberate or accidental. Non-native species can have various effects on the local ecosystem. Introduced species that become established and spread beyond the place of introduction are called invasive species. Some have a negative effect on a local ecosystem. Some introduced species may have no negative effect or only minor impact. Some species have been introduced intentionally to combat pests. They are called biocontrols and may be regarded as beneficial as an alternative to pesticides in agriculture for example. In some instances the potential for being beneficial or detrimental in the long run remains unknown. A list of some introduced species is given in a separate article.