A flat-file database is a database stored in a file called a flat file.
Records follow a uniform format, and there are no structures for indexing or recognizing relationships between records.
The file is simple. A flat file can be a plain text file (e.g. csv, txt or tsv), or a binary file.
Relationships can be inferred from the data in the database, but the database format itself does not make those relationships explicit.
The term has generally implied a small database, but very large databases can also be flat.