Terminology on biological invasions is very complex and often inconsistent. Apart from „alien”, the species in question are referred to as „introduced”, „acclimatized”, „invasive”, „non-indigenous”, „allochtonous”, or „exotic”.

„Alien Species in Poland” follows definitions included in the Regulation (EU) No 1143/2014 of the European Parliament and of the Council of 22 October 2014 on the prevention and management of the introduction and spread of invasive alien species.

Alien species means any live specimen of a species, subspecies or lower taxon of animals, plants, fungi or microorganisms introduced outside its natural range; it includes any part, gametes, seeds, eggs or propagules of such species, as well as any hybrids, varieties or breeds that might survive and subsequently reproduce.

Invasive alien species means an alien species whose introduction or spread has been found to threaten or adversely impact upon biodiversity and related ecosystem services.

Introduction means the movement, as a consequence of human intervention, of a species outside its natural range.