One of the goals of the WG1 (THEORY) of this COST Action was an extensive literature review of existing theory and research on opinions and their textual/discursive expression that would allow us to develop a glossary of key definitions and terms.We aim in offering an interdisciplinary, still coherent, overview of key terms and definitions to scholars who have been conducting studies on opinion or are planning to start doing that. This Glossary is an outcome of collaborative work of more than 30 scholars from 13 countries and regions. The Glossary contains over 140 entries, each providing a clear definition,highlighting the main features of the concept or phenomenon, and outlining its theoretical background. Each entry concludes with a selection of references for further reading. All entries have been peer-reviewed and edited into a standardisedformat. We hope this Glossary will serve as a valuable resource for researchers and practitioners alike, fostering a shared understanding of key concepts and facilitating interdisciplinary dialogue on the study of opinions. By clarifying definitions and theoretical foundations, we aim to contribute to more rigorous, transparent, and collaborative research in the fields of linguistics, communication, and social sciences. We warmly thank all contributors and reviewers for their dedication to thiscollective effort.