When a beam of white light is allowed to pass through convex lens parallel to principle axis, the different colors of light converge at different point on the principle axis after refraction. This is called

The refractive index of the material of a lens varies with the wavelength of light. According to Cauchy's formula, the refractive index is different for different colors of white light.

The focal length of a lens is given by the lens maker's formula: $\dfrac{1}{f} = (\mu - 1)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$.

Since $\mu$ is different for different colors, the focal length $f$ will also be different for different colors. As a result, when a beam of white light passes through a convex lens parallel to the principal axis, the different colors converge at different points on the principal axis.

This inability of a lens to focus all colors of light at a single point is known as chromatic aberration.

Answer: chromatic aberration