Abstract: Invasive alien species are major threats to biodiversity around the world, and China is one of the countries that suffers from the most severe invasions of alien species. The definition of invasive alien species is a controversial topic, yet the first step in preventing and controlling invasive alien species is to accurately define them and clear control objects. In this paper, the methods used for defining invasive alien species in various conventions and in different countries and regions are comprehensively analyzed. The main challenges of defining invasive alien species in terms of expression, spatial scope, time range, influence, and hierarchy are then discussed. By analyzing the concept and connotation of invasive alien species, the important issues surrounding invasive alien species are explored and new reference ideas are provided for lawmakers working on prevention and control policies.