Abbreviation: SD = "Super Low Dispersion" (Tokina)

The abbreviations means "Super Low Dispersion"

Explanation from Tokina

When standard optical glass is used in telephoto lenses, a phenomenon called chromatic aberration can occur. Chromatic aberration is the inherent tendency for glass to disperse (separate) a ray of light into the colors of the rainbow. The rainbow effect created by a glass prism is the most dramatic demonstration of chromatic aberration. In lenses, it is much less pronounced, but still creates slightly out of focus colors, akin to an “optical noise” that has a negative impact on the quality of the picture. To eliminate chromatic aberration, Tokina employs expensive, special glass material having super- low dispersion (SD) properties.

Lenses in the Tokina line-up with the SD mark incorporate these Super-Low Dispersion glass elements, minimizing the secondary spectrum or optical noise caused by chromatic aberration.

See examples of effected products

Tokina AT-X 16-28mm f/2,8 PRO FX
Tokina AT-X Pro 16-50mm f/2,8 DX

See links

tokinalens.com