The alliance is the basic unit of the EuroVegChecklist, and it often serves as the lowest rank in broad-scale vegetation surveys. However, there is hardly any literature about the concept and definition of this syntaxonomic rank, leading to uncertainty in its application. Here, I explore the original association concept of Braun-Blanquet, which was based on absolute character species, and I show that this concept is more or less identical with the units that we now call alliances. By also incorporating the concept of central syntaxa, I propose the following definition: “An alliance is a moderately broad vegetation unit that either has one or several absolute character taxa or that can be interpreted as the central alliance of an order.” The one-to-one relationship between character taxa and vegetation units gives the latter a clear biogeographical and evolutionary meaning. Restrictions to the validity of character taxa – either to certain geographical areas or to physiognomic types – cause theoretical and practical problems and should be avoided. Possible exceptions are species with similar frequency in two formations or species introduced to other continents.<br> Taxonomic reference: Euro+Med PlantBase (http://ww2.bgbm.org/EuroPlusMed/) [accessed 1 July 2020].
