Red Dot
A non-magnifying optic projecting an illuminated reticle onto a lens.
Also: RDS, reflex sight
A red dot sight (RDS, or reflex sight) projects an illuminated dot onto a lens that the shooter looks through. Both eyes can stay open; the dot appears superimposed on the target. Red dots are faster than iron sights for most shooters at typical defensive distances and have become the standard duty/defensive optic on rifles and increasingly on pistols.