What is Meant by "Dominant Eye" or "Master Eye" in Shooting?

How to Determine Dominant Eye Master Eye
To determine dominant or master eye, hold hands out like so. With both eyes open, center an object in the opening between hands. Now, close left eye. If you can still see the object, your right eye is dominant; if you cannot, left eye is dominant. Photo © Russ Chastain

In most people, one eye is dominant, meaning that the brain exhibits a neurological preference for the visual input from that eye.  (Technically, this is known as "ocular dominance.") The dominant eye is usually (but not always) the right eye for right-handed people and the left eye for left-handed shooters. In a few cases, there is no preference for one eye over the other, and such individuals are said to be  cross-dominant.)

How do you tell which eye is dominant?

For shooters with two eyes of fairly equal vision, you can determine your dominant or master eye by holding your hands out in front of you at arm's length, forming an opening between your hands as shown in the photo. With both eyes open, center an object in the opening between your hands. Now, close your left eye. If you can still see the object, your right eye is dominant; if you cannot, then your left eye is dominant.

The dominant eye is important because that is the eye that your brain automatically "wants to" use when aiming a gun. Realizing which eye is dominant can be very helpful in deciding how you should practice and aim. A right-handed person with a dominant left eye may end up doing everything else right-handed, but will shoot a gun left-handed. A shooter normally aims using the dominant eye, holding the non-dominant eye closed. 

If you find your eyes fairly equal in dominance, you should shoot with your strong hand (right for right-handed folks) and use that eye for aiming, closing or squinting the other eye when aiming.