Taxonomists classify biological specimens into groups (taxa) on the basis of similarities between their observed features ('characters'). The description of these 'characters' is therefore central to taxonomy, but there is currently no agreed model, defined terminology nor methodology for composing these descriptions. This lack of a common conceptual model, together with the individualistic working practices of taxonomists, means that descriptions are not composed consistently, and are not easy to interpret and re-use, nor are datasets comparable. The purpose of the Prometheus II project is to improve the interpretation and comparison of plant descriptions. To this end we propose a new conceptual model for unambiguously representing character descriptions, and have developed a controlled vocabulary as an ontology of defined terms, which will be used to describe specimens according to our character model.