Abbreviation: AD = "Anomalous Dispersion" (Tamron)

The abbreviation AD means "Anomalous Dispersion" and is seen on Tamron products.

Explanation from Tamron

Anomalous Dispersion (AD) for Better Color Correction

Anomalous dispersion (AD) glass is a special type of optical glass that is used to achieve more precise control of chromatic aberrations, thereby enhancing overall imaging performance. Glass of this type provides an abnormally large partial dispersion ratio (amount of diffraction) for light of specific wavelength ranges (colors) within the visible spectrum. By combining AD glass having these special characteristics with elements made of normal glass having different dispersion characteristics, it is possible to control the dispersion factors of a specific wavelength. This enhanced level of control results in much lower levels of on-axis (central) chromatic aberration for telephoto lenses (or zooms used at tele-photo settings) and a significant reduction of lateral (peripheral) chromatic aber-ration for wide-angle lenses (or zooms used at wide-angle settings.)

See examples of effected products

See links