Abstract

The organism is the central entity in biological science. However, consensus with regard to the definition of the underlying concept is lacking. Moreover, several ambiguous life forms exist that challenge current definitions of the term. Based on a comprehensive analysis of the available literature, we provide an overview of the criteria and approaches that have been previously used to define organismality. In addition, we highlight non-paradigmatic biological entities to identify problems that challenge definitions of organismal units. To address these issues, we propose a cross-disciplinary definition of the organism concept and provide a list of key criteria that allow both paradigmatic and non-paradigmatic cases to be unequivocally classified. In this way, our work not only provides newcomers with an overview of this exciting field, but also enhances communication across disciplines.