Achromatic refers to something that does not have color or is devoid of color. It's often used in the context of optics, describing lenses or prisms that are designed to minimize chromatic aberration, which is the blurring of colors caused by different wavelengths of light refracting at different angles.
Therefore, an achromatic lens is one that refracts all colors of light at the same angle, resulting in a clearer and sharper image.
If you were referring to something else, please clarify and I will be happy to help!