Constant Linear Velocity.  A DVD (other than DVD-RAM) is composed of a spiral track of tiny pits.  The player reads the track of pits by focusing a laser beam on the track, and scanning along the track at a constant linear velocity, measured in meters per second.  As the disc reads along the spiral track, the radius of the track gradually increases, or in the case of layer 1 of a OTP DVD-9, decreases.  Therefore, the rotation speed (angular velocity) must change in order to maintain a constant linear velocity.
Modern players are able to increase or decrease this speed as necessary.  DVD-ROM drives are able to increase the reading speed to multiples of the original standard speed (1x, 2x, 4x, 6x, 8x, 12x, etc).  DVD Burners are also able to write at faster multiples of the standard linear velocity.  Nonetheless, the linear velocity is constant.  Compare to Constant Angular Velocity.