Abbreviation: DC = "DC for Digital" (Sigma)

The abbreviations means "DC for Digital"

Explanation from Sigma

These are special lenses designed so that the image circle matches the smaller size of the image sensor of most digital SLR cameras. Their specialized design gives these lenses the ideal properties for digital cameras, the compact and lightweight construction is an added bonus ! including compact and lightweight construction.

These are dedicated digital SLR camera lenses with an image circle designed to suit image sensors that correspond to APS-C size. Inhouse technology accumulated through the development of our digital SLR camera was used to optimize optical performance for our digital lenses. This is a high-performance lens series that fuses technology such as lens power layout and coating design with the know-how Sigma has built up over many years of developing interchangeable lenses for SLR cameras. By reducing the diameter of the image circle, Sigma has achieved a more compact, lightweight lens series.

See examples of effected products

Sigma EX 50-150mm f/2,8 DC APO HSM
Sigma 8-16mm f/4,5-5,6 DC HSM
Sigma 17-70mm f/2,8-4 DC HSM OS Macro

See links

sigmaphoto.com
sigma-photo.co.jp