SQLite
Embedded, file-based SQL database — the most-deployed database in the world, runs without a server process.
Definition
SQLite stores an entire database in a single file and runs in-process with the application; no daemon, no network. It backs every iOS and Android app, most desktop apps, and Ghost's default install. It is the right default for any single-writer workload up to tens of GB. Concurrent writes are serialised, which makes Postgres the better choice under multi-writer load.