Biodiversity is undergoing a major crisis. Institutions, while launching initiatives tackling the issue, are using and diffusing the term biodiversity and related expert knowledge. However, to collectively address the biodiversity crisis, it is important that actors are able to communicate with each other. This is particularly true in the three-part set including science, public institutions, and citizens. In this paper, we explored this mutual understanding with a focus on laypeople: we assessed the understanding of biodiversity in a sample of 1209 French adult citizens and explored the convergences and divergences with institutional and academic definitions. With a classical hypothetical-deductive approach, we first showed an overall congruence between laypeople and institutions: 80% of respondents provided a descriptive definition of plant and animal species as well as their diversity, which are main ideas diffused by institutions. However, based on the high diversity of the collected definitions, with 57% of provided words in definitions mentioned only once, we complemented this study with an inductive approach. We showed a discrepancy in the definitions from lay people and from conservation science (based on evolutionary and dynamic processes). We also highlighted that 18,5% of definitions are not descriptive and are referring to specific actions for biodiversity conservation. We discuss these results in the context of social-ecological transitions, and encourage conservation communities to acknowledge the range of biodiversity definitions used by laypeople, and to form closer relationships with laypeople to anchor conservation research and action with a bottom-up dynamic process of knowledge sharing.