SQLite is a compact open source database that is included with most of modern computers and smartphones (iOS, Android, Windows, MacOS) even in Chrome and Firefox web browsers.
It is also good for in-process data caching.
Most Widely Deployed SQL Database Engine
"Since SQLite is used extensively in every smartphone, and there are roughly 3.5 billion smartphones in active use, each holding hundreds of SQLite database files, it is seems likely that there are over one trillion SQLite databases in active use. Most Widely Deployed Software Module of Any Type?"
Well-Known Users Of SQLite
SQLite in 2018: A state of the art SQL dialect
"SQLite is an undervalued database—some people even believe it is a toy database that is not suitable for production use. In fact, SQLite is a very solid database that can handle terabytes of data.”
"An SQLite database is limited in size to 140 terabytes (247 bytes)"
