Indeed, the definition of dvd is digital video disc or digital versatile disc and it is an optical disc storage medium. A red laser is used to read the discs.
Blu-ray disc (known also as BR, BD, or simply Blu-ray) is also an optical disc storage medium, but uses a blue-violet laser to read the media.
Therefore, it would be correct to call them both optical discs but it's a misnomer to refer to a Blu-ray disc as a dvd.
For the record, Compact Discs (CDs) are also an optical disc storage medium. You wouldn't call a Blu-ray disc a Blu-ray CD, would you?
As for where to find cheap discs, I agree wholeheartedly with tboy. In fact, I've never even heard of redflagdeals.com so thanks for that.