The experimental characteristics of divertor detachment in the JET tokamak with the Mark I pumped divertor are presented for Ohmic, L-mode and ELMy H-mode experiments with the main emphasis on discharges with deuterium fuelling only. The range over which divertor detachment is observed for the various regimes as well as the influence of divertor configuration, direction of the toroidal field, divertor target material and active pumping on detachment will be described. The observed detachment characteristics such as the existence of a considerable electron pressure drop along the field lines in the scrape-off layer, and the compatibility of the decrease in plasma flux to the divertor plate with the observed increase of neutral pressure and the Dα emission from the divertor region will be examined in the light of existing results from analytical and numerical models for plasma detachment. Finally, a method to evaluate the degree and window of detachment is proposed and all the observations of the JET Mark I divertor experiments summarised in the light of this new quantitative definition of divertor detachment.