Chromium is the open source project that powers Google's Chrome browser. Because the project is open source, both Google and others can build upon and use the Chromium source code. Anyone can download, compile, and tinker with the Chromium source code.
Google Chrome vs. Chromium – What’s the Difference?
As a web browser, Chromium is inherently less stable than Chrome, meaning it crashes more often and can exhibit other types of unwanted behavior. However, it is compatible with Chrome browser extensions, and offers a very similar user experience without subjecting itself to Google’s invasive data collection.
Chromium and Chrome are closely related. Simply put, Chrome is almost entirely based on Chromium. When Google releases a new version of Chrome, they take stable code from the Chromium project and add their own proprietary code to implement features like automatic updates.
Chrome is basically just Chromium with some extra features. Chromium is the primordial soup from which Chrome was created.