The Drop is a system for defining the fit of a jacket or suit jacket. Simply put it is the difference between the measurement of the chest and the waist.
For example a European size 50 Drop 7 with have a waist size of 43cm based on half of the same trousers. The higher the Drop (plus 8/9) the more the likelihood of the suit or jacket requiring a slim fit as the difference between chest and waist measurements is greater.