This document attempts to represent a wide range of opinions in an inclusive way, to allow for different types of projects and programmes, where context-specific criteria can be set.The characteristics outlined below are based on views expressed by researchers, practitioners, public officials and the wider public. Our aim is to identify the characteristics that should be considered when setting such criteria (e.g. a funding scheme), and we call upon readers to determine which subset of these characteristics is relevant to their own specific context and aims. These characteristics build on (and refer to) the ECSA 10 principles of citizen science as a summary of best practie – and projects are expected to engage meaningfully with them. Where it is especially pertinent, we refer to them in the characteristics below. The rest of the document covers the characteristics of citizen science under five sections: (1) core concepts; (2) disciplinary aspects; (3) leadership and participation; (4) financial aspects; and (5) data and knowledge. Further explanation and background are provided in the ‘ECSA’s characteristics of citizen science: explanation notes’ document. The research article describing this work 'Contours of citizen science: a vignette study' can be found in the Royal Society Open Science journal at https://doi.org/10.1098/rsos.202108.

The characteristics working group included Muki Haklay, Alice Motion, Bálint Balázs, Barbara Kieslinger, Bastian Greshake Tzovaras, Christian Nold, Daniel Dörler, Dilek Fraisl, Dorte Riemenschneider, Florian Heigl, Fredrik Brounéus, Gerid Hager, Katja Heuer, Katherin Wagenknecht, Katrin Vohland, Lea Shanley, Lionel Deveaux, Luigi Ceccaroni, Maike Weisspflug,, Margaret Gold, Marzia Mazzonetto, Monika Mačiulienė, Sasha Woods, Soledad Luna, Susanne Hecker, Teresa Schaefer, Tim Woods and Uta Wehn. The development of these characteristics was supported by European Union's Horizon 2020 research and innovation programme under grant agreement No. 824580, project EU-CiƟzen.Science (The Platform for Sharing, Initiatng, and Learning Citizen Science in Europe), the ERC Advanced Grant project European CiƟzen Science: Analysis and Visualisation (under Grant Agreement No 694767). Thanks to the Bettencourt Schueller Foundation long term partnership, this work was partly supported by CRI Research Fellowships to Muki Haklay, Alice Motion, and Bastian Greshake Tzovaras The research article describing this work 'Contours of citizen science: a vignette study' can be found in the Royal Society Open Science journal at https://doi.org/10.1098/rsos.202108.