SQLite is an embedded SQL database engine created by D. Richard Hipp. Its own documentation describes it as “an in-process library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine.” Unlike most SQL databases, SQLite has no separate server process: a complete database with tables, indexes, triggers, and views lives in a single ordinary disk file, and the library reads and writes that file directly. The project advises thinking of SQLite “not as a replacement for Oracle but as a replacement for fopen().”
Because it is tiny — under roughly 900 KiB with all features enabled — and requires no setup, SQLite gets linked directly into applications rather than installed as a service. SQLite’s “most deployed” page argues it “is likely used more than all other database engines combined,” noting that it ships in every Android, iPhone, and Mac device, every Windows 10 and 11 install, all major web browsers, and countless applications. With billions of smartphones each holding hundreds of database files, the project estimates there are likely over one trillion SQLite databases in active use.
SQLite is also notable for its license: rather than an open-source license, its code and documentation are dedicated to the public domain, with contributors signing affidavits to that effect. The combination of small size, single-file storage, reliability, and frictionless licensing is what made SQLite the default embedded database almost everywhere software runs.