The necessity for robust, enduring, and relevant healthcare interoperability is universal across all clinical domains. However, we identified a gap in the availability of open-source, no-cost, high-quality tools that offer multilingual support and an advanced graphical interface. To address this, we developed TermX, an open-source platform to harmonise terminology and support interoperability between healthcare institutions and systems. TermX incorporates a terminology server, a Wiki, a model designer, a transformation editor, and tools for authoring and publishing. TermX is designed to develop terminology and implementation guides for healthcare systems at both the national and regional levels. It aims to ensure open, standardised access to published data and guarantee semantic interoperability based on the FHIR standard.