The process and means of extracting data from a database for use elsewhere.
Different from a backup, a data export is usually an extraction of a subset of data from a database, normally for use elsewhere. The export may be done through a query or reporting tool, a bespoke application, or some systems ship with general purpose data import/export tools.
A backup is a specific format designed for disaster recovery, and is not designed to be manipulated by third parties. Exporting data normally implies that it is intended to be used outside the database, perhaps by loading it into a different database or manipulating it client-side with an application such as a spreadsheet.