In computing, a database is an organized collection of data stored and accessed electronically from a computer system. Where databases are more complex they are often developed using formal design and modeling techniques.
Or to put it another way, it’s a bunch of data, probably stored on a computer or some other place.
With such a broad definition almost anything can be considered a database.
Retrofitting MongoDB over something like PostgreSQL is also worth considering if you have minimal touch points and don’t have exotic column lookup requirements.