Red Dot Sight

A Red Dot Sight is a reflex sight for most two handed weapons that gives a nearly unobstructed view of the shooter's target unaffected by parallax or distance while giving them a crosshair from which to shoot. This makes it regarded as a fast, easy to use and versatile sight for police, military and hunting purposes alike.

Battlefield 2
A red dot sight appears on the AKS-74u, M4A1, SCAR-L, SCAR-H, and P90 weapons as an unremovable sight. Although the frame of the sight is different for each weapon (a Kobra sight for the AKS-74u, an Aimpoint M68 sight for the M4A1, and an FN micro red dot for the P90), the actual dot is the same amongst all of the different sights.

Battlefield 3
The Red Dot Sight is appears as an attachment in Battlefield 3. The regular RDS, named the Reflex (RDS), is unlocked for weapons used by the United States Marine Corps and the Kobra can be unlocked for weapons used by the Spetsnaz. Functionally the two sights are identical, and can be unlocked for the opposite faction's weapons when all the faction default attachments have been unlocked for a weapon. The Red Dot Sight in Battlefield 3 seems to be the Trijicon RX01 Variant.

Trivia

 * In Battlefield 2, the PLA QBZ-97 does not come with a red dot sight, making it the only Special Forces default weapon to not have a red dot.