A single-file, lightweight SQL database platform
SQLite is data and the database software all wrapped into one file. The database software portion is less than 500kB, so it doesn’t add a ton of overhead to your files, while retaining features over CSV such as Referential Integrity and indexing.
SQLite is Public Domain, but the code is marked as “Open-Source, not Open-Contribution”. Basically they don’t accept patches or updates from people unless those people sign affidavits saying they dedicate their contribution to the public domain.
SQLite is used everywhere, according to Simon Willison, the guy behind Datasette.