Unless you're a DBA, you'd probably define a database backup as a complete copy of a database at a given point in time. While that's one type of database backup, there are many others. Consider a multi-terabyte database that's used 24/7:
How long does the backup take, and what impact does it have on users?
Where are the backups stored, and what is the media cost?
How much of the database changes each day?
If the database failed partway through the day, how much data would be lost if the only recovery point was the previous night's backup?
In considering these questions, particularly for large databases with high transaction rates, we soon realize that simplistic backup strategies limited to full nightly backups are insufficient ...