The community of editors, developers, integrators, documentation, open-source code, training resources, blog posts, forums, consultants, and other users that forms around the usage and extension of a particular CMS. The ecosystem provides peer support, code, plug-ins, and discussion around aspects, benefits, and challenges of the CMS.
Communities grow over time and with the growth of the product. Additionally, open source products tend to have larger communities than commercial products due to a larger installed base and more open philosophy of users and system integrators.