A destination wedding is any wedding held somewhere most of the guests have to travel to. That is the working definition; it covers everything from a 20-guest elopement in a Tuscan agriturismo to a 150-guest full resort buyout in Cabo. The common thread is that the location is the decision, and every other logistical choice bends around it.
In practice, three things distinguish a destination wedding from a local one. First, time: the weekend stretches to three or four nights because the travel makes a one-day event impractical. Second, guests: some people will decline who would have come to a local wedding, and some who rarely travel will come because this is the reason. Third, logistics: a layer of coordination you do not have locally (flights, hotel blocks, transfers, welcome events) sits on top of the standard planning.