What the heck does APO mean

Sigma 70-300mm f/4-5.6 DG APO Macro Telephoto Zoom Lens for Canon SLR Cameras
This is what I see, but I can't for the life of me figure out what APO means. Also, Whats the difference between that one and this one....
Sigma 70-300mm f/4-5.6 DG Macro Telephoto Zoom Lens for Canon SLR Cameras
Both on amazon, but the one without the APO in it is 70 dollars less.
Help??
Jason

Definition I found

Apochromatic (APO lens)
Lens corrected for chromatic aberrations in all three primary colours.

another

APOCHROMATIC - often shortened to "APO," means corrected for spherical and chromatic aberration. Lenses that are apochromatic cause all visible light wavelengths to focus on the film plane.

APO means that the lens was made with some kind of glass to reduce/minimize color aberration.

The lens without APO is probably less because you might get color aberration with it (only a guess).

this was actually asked in a thread in the digital slr section earlier this week ... i found this and posted it there also

http://www.sigmaphoto.com/lenses/lenses.asp#apo

:thumbsup:

Yep, that's how it works. This isn't the only thing that contributes to image sharpness, but it wouldn't hurt. Would it help enough to justify it? Can't answer on that particular lens but you might want to check the reviews. From what I know, it's most beneficial with long focal lengths. I had a manual focus Nikon 300 f4.5 ED lens (ED is the same idea as APO, lots of manufacturers have it). It was also available as a non-ED lens but mine was one of the best lenses I've ever had. I've heard mixed reviews about the non-ED but never had a chance to compare them side by side.

Thank you very much, your comments actually helped quite a bit.
Thanks Again,
Jason