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 agreed by the Convention on Biological Diversity (CBD) and then implemented in the "European Strategy on Invasive Alien Species" adopted by the Standing Committee of the Bern Convention.

Alien species: a species, subspecies or lower taxon, introduced outside its natural past or present distribution; includes any part, gametes, seeds, eggs, or propagules of such species that might survive and subsequently reproduce.

Invasive alien species (IAS): an alien species whose introduction and/or spread threaten biological diversity. (For pragmatic reasons, in the database this group also includes alien species whose introduction and/or spread threaten economy and/or human health ).

Introduction: the movement by human agency, indirect or direct, of an alien species outside of its natural range (past or present). This movement can be either within a country or between countries or areas beyond national jurisdiction.