chromatic aberration short handed for CA
"CA is most evident anytime you're viewing high contrast surfaces juxtaposed against one another. It is seen as a color fringe (usually yellow or purple, in my experience) along the edges of dark objects against a light background, or vice-versa. The degree to which you see CA also depends on how defined the high contrast edge between dark and light is, and how much ambient light is present. It doesn't matter whether the background is sky, snow, or whatever, it's merely an issue of high contrast, dark against light.
Some CA is present in most optics to varying degrees, simply because different wavelengths (colors) of light don't converge at the same focal plane when light passes through lenses, however optics with ED (extra-low dispersion... also sometimes referred to as HD, FL, APO, etc.) glass types, lens elements, or optical design minimize this dispersion so that CA is effectively reduced to the point it isn't noticeable in all but the most severe conditions.
Does it matter? That depends on how picky you are, what the intended use of the optic is, how much fine detail you wish to discern in normal viewing, and how sensitive your eyes are in seeing CA. I don't like excessive CA, and I can readily see it. CA reduces apparent sharpness/resolution, so, for example, if you wish to evaluate fine antler detail like kicker points on deer, CA can reduce your ability to see small details in certain circumstances. However, it honestly doesn't inhibit the usefulness of the optic the majority of the time, other than the image quality isn't as sharp and pleasing to the eyes as it would be with CA well-controlled. Low CA is much less important in a rifle scope than it is in binoculars, where you're doing a lot of glassing. Having absolutely no CA whatsoever under any circumstances is neither essential nor technically possible.
I look at it this way: CA, like any all aberrations, reduces image quality. All else being equal, I want the least amount of optical aberrations possible without having to pay a fortune to get it. Realizing there are tradeoffs between the good and bad in everything and that there's no such thing as absolute perfection in any optic, I'm willing to live with some small amount of CA if the optic is excellent in other respects, but low CA is one of my "wants" in any optic. I'm much more picky about image quality in binoculars, which are used to search for and evaluate things, and which I spend way more time peering through than I am in a riflescope, which is first and foremost an aiming device. Different people have different priorities. The only way to know if a given optic meets your expectations for CA or any other undesirable optical trait is to spend time looking through it."
I copied it and pasted here from a guy wiser than I