Abbreviation: D = "Distance information" (Nikon)

The abbreviations means "Distance information"

Explanation from Nikon

D-type and G-type NIKKOR lenses relay subject-to-camera distance information to AF Nikon camera bodies. This then makes possible advances like 3D Matrix Metering and 3D Multi-Sensor Balanced Fill-Flash. Note: D-type and G-type NIKKOR lenses provide distance information to the following cameras: Auto exposure; F6, F5, F100, F90X, F80, F75, F70, F65, F60, F55, F50, PRONEA S, PRONEA 600i, D2 series, D1 series, D100 and D70s/D70. Flash control; F6, F5, F100, F90X, F80, F75, F70, D2 series, D1 series, D100 and D70s/D70.