Abbreviation: APO MACRO = "Apochromatic Macro" (Sigma)

The abbreviations means "Apochromatic Macro"

Explanation from Sigma

Although telephoto zoom lensescan be used closer to the objectthan fixed focal length telephotolenses, there is still a minimumshooting distance. SIGMA hasmade this minimum distance smaller and developed the zoom MACRO lensfor taking close-up photographs of the same quality as those taken with aregular MACRO lens, while maintaining the performance specific to anAPO lens. Rather than carrying around the cumbersome accessoriesrequired for close-up work, the photographer can now take photographs ata magnification of 1:2 (one halflifesize) using a telephoto lens, byquickly shifting from the normalsetting to the full macro setting.

See examples of effected products

Sigma 70-300mm f/4-5,6 DG APO Macro
Sigma EX 70-200mm f/2,8 II APO HSM DG Macro
Sigma EX 150mm f/2,8 DG HSM APO Macro

See links

sigma-photo.co.jp