Emmetropia is the zero refractive state in which an eye is neither nearsighted, farsighted, nor astigmatic. This causes the natural focal plane to be full correction. An eye is said to be emmetropic when the image of an object at infinity (parallel light rays) forms on the retina, rather than in front of or behind it, implying it is also free of astigmatism.
Eyes are generally regarded as emmetropic if their spherical equivalent is between -0.5 and +0.5 D and can achieve 20/20 distance visual acuity without lenses.
This permits some myopia, hyperopia, and astigmatism. However, a truly emmetropic eye would typically achieve 20/8 acuity (no myopia or astigmatism) and be blurred to 20/20 through a +0.25 lens (no hyperopia).