Abbreviation: LD = "Low Dispersion" (Tamron)

The abbreviations means "Low Dispersion"

Explanation from Tamron

Low Dispersion (LD) Glass for Greater Lens Sharpness
Low dispersion (LD) glass elements in a lens help reduce chromatic aberration; the tendency of light of different colors to come to different points of focus at the image plane. Chromatic aberration reduces the sharpness of an image, but glass with an extremely lowdispersion index, has less of a tendency to separate (defract) a ray of light into a rainbow of colors. This characteristic allows the lens designer to effectively compensate for chromatic aberration at the center of the field (on axis), a particular problem at long focal lengths (the telephoto end of the zoom range), and for lateral chromatic aberration (towards the edges of the field) that often occurs at short focal lengths (the wide-angle end of the zoom range.)

See examples of effected products

Tamron SP AF 17-50mm f/2,8 XR Di-II LD Asph IF
Tamron SP AF 180mm f/3,5 Di LD IF Macro
Tamron AF 55-200mm f/4-5.6 Di II LD Macro

See links

tamron-usa.com