Red Dot Sight
Also known as: reflex sight
A non-magnifying electronic sight that projects an illuminated aiming dot, allowing fast target acquisition with both eyes open.
A red dot sight projects a small illuminated reticle (usually a dot) onto a lens, so you simply place the dot on the target and fire. Because there's no magnification and the dot floats on the target plane, you can shoot with both eyes open for excellent situational awareness and very fast target acquisition. Common types include tube-style sights and open 'reflex' sights.
Red dots have largely taken over for close-to-mid-range use on rifles, shotguns, and increasingly pistols. The dot is parallax-free for practical purposes, so minor head position changes don't shift your point of aim much. The main considerations are battery life (leave it on), zeroing the dot to your load, and choosing an appropriate dot size — measured in MOA.