The ZIM file format is an open file format that stores website content for offline usage. The format is defined by the openZIM project, which also supports an open-source ZIM reader called Kiwix. The format is primarily used to store the contents of Wikipedia and other Wikimedia projects, including articles, full-text search indices and auxiliary files.
ZIM is an abbreviation for 'Zeno improved', as it replaced the earlier Zeno file format. Since 2021, the library defaults to Zstandard file compression, and also supports LZMA2 compression, as implemented by the XZ Utils library. Most notably, the compression ratio can be up to 3∶1 with almost all of the space savings taking place within the clusters.
The openZIM project is supported by the Wikimedia Foundation.