Chromatic aberration is the term used to describe the blurring of an image the occurs when an object is formed by a lens from a range of wavelengths. The refractive index of glass varies with wavelength, so each wavelength of light will be refracted by a slightly different amount.
Blue light is refracted more than red light in the image below, so an image formed by blue light will be formed closer to the lens than the image formed by red light.
The effect can be eliminated for any two colours by using a compound lens, called an achromatic doublet, made of different types of glass.. The two lenses, for a particular wavelength, produce equal and opposite distortion. Though the effect is eliminated for only a single wavelength, it is reduced for all.