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

The abbreviations means "DG for Digital"

Explanation from Sigma

These are large-aperture lenses with wide angles and short minimum focusing distances. With an abundance of peripheral illumination, they are ideal lenses for Digital SLR Cameras whilst retaining suitability for traditional 35mm SLRs.

These are high-performance lenses optimized for digital cameras. They are ideal not only for 35 mm digital SLR cameras and film digital SLR cameras but also for APS-C digital SLR cameras. They deliver superior image quality by correcting for all types of aberration, especially distortion, and they have high resolution capability because they minimize chromatic aberration of magnification, which is particularly a problem for digital cameras. What is more, these lenses incorporate optical designs optimized for digital cameras and Sigmas own Super Multi-Layer Coating technology, so they reduce flare and ghosting due to reflection between the image sensor and lens surfaces, and they provide high contrast and a wide range of tone. They also provide enough peripheral brightness with little vignetting.

See examples of effected products

Sigma EX 15mm f/2,8 DG Fisheye
Sigma EX 24-70mm f/2,8 DG HSM
Sigma EX 70-200mm f/2,8 APO HSM DG OS

See links

sigmaphoto.com
sigma-photo.co.jp