An online appendix is a supplementary document that accompanies a research paper or study and provides additional information or data that supports the main findings or arguments in the main paper. This appendix is typically made available online and is not included in the printed version of the research paper. It may include detailed methodology, additional results, tables, figures, or other supporting materials that were not included in the main paper due to space constraints. Researchers often use online appendices to provide transparency and ensure that readers have access to all relevant information related to the study.