Refractive index of any medium varies with wavelength other than vacuum and the refracting properties of optical system are function of wavelength. Hence, chromatic aberration exist. 
Definition of chromatic aberration:
Chromatic aberration is, “Failure of optic system to bring all colors to a common focus“. The other name for chromatic aberration is color aberration.
If there is no chromatic aberration, then all the rays with different color properties will pass through the center focus.
Types of chromatic aberration:
1) Longitudinal chromatic aberration (axial):
Longitudinal chromatic aberration occurs because of change in focusing distance or back focal length.
2) Transverse chromatic aberration (lateral):
Transverse chromatic aberration occurs because of change in magnification.
 W T Welford FRS, “Aberrations of optical systems”, Adam hilger imprint by IOP publishing Ltd, ISBN 0852745648, 1991.
 James C Wyant, Katherine Creath, “Applied Optics and Optical Engineering”, Academic Press Inc, Vol. XI, pp. 0339, ISBN 012408611X, 1992.