Dropbox doesn't delete your files when you delete them from dropbox. You can always restore them later, I have used that feature countless of times. So Dropbox can act like a backup, I still use JungleDisk to backup my whole dropbox just to be safe.
Backup to me (and most others that I've talked to) means "I can restore all my data, my entire hard drive, and be back to where I was yesterday, so if I screw up my kernel tonight, I can just use the backup to revert to my pre-kernel hacking spree".
(to an average joe, it means the same thing, but not as much detail)
