Ecological research produces a tremendous amount of data, but the diversity in scales and topics covered and the ways in which studies are carried out result in large numbers of small, idiosyncratic data sets using heterogeneous terminologies. Such heterogeneity can be attributed, in part, to a lack of standards for acquiring, organizing and describing data. Here, we propose a terminological resource, a Thesaurus Of Plant characteristics (TOP), whose aim is to harmonize and formalize concepts for plant characteristics widely used in ecology. 2. TOP concentrates on two types of plant characteristics: traits and environmental associations. It builds on previous initiatives for several aspects: 1) characteristics are designed following the entity-quality model (a characteristic is modelled as the “Quality” of an “Entity” ”) used in the context of Open Biological Ontologies; 2) whenever possible, the Entities and Qualities are taken from existing terminology standards, mainly the PO (Plant Ontology) and PATO (Phenotypic Quality Ontology) ontologies, and 3) whenever a characteristic already has a definition, if appropriate, it is reused and referenced. The development of TOP, which complies with semantic web principles, was carried out through the involvement of experts from both the ecology and the semantics research communities. Regular updates of TOP are planned, based on community feedback and involvement. 3. TOP provides names, definitions, units, synonyms and related terms for about 700 plant characteristics. TOP is available online (www.top-thesaurus.org), and can be browsed using an alphabetical list of characteristics, a hierarchical tree of characteristics, a faceted and a free-text search, and through an Application Programming Interface. 4. Harmonizing definitions of concepts, as proposed by TOP, forms the basis for better integration of data across heterogeneous data sets and terminologies, thereby increasing the potential for data reuse. It also allows enhanced scientific synthesis. TOP therefore has the potential to improve research and communication not only within the field of ecology, but also in related fields with interest in plant functioning and distribution