What is swarm in Pokemon Diamond?

A swarm, in Pokemon terminology, is a group of unobtainable, or rare Pokemon in a certain area, in bulk. These groups of Pokemon appear randomly every day. In Pokemon Diamond you can determine the swarm of the day by going to the house south of Rowan's lab in Sandgem town, the girl in there will tell you where the swarm is, and what Pokemon it is consisted of. For Swarms to appear you must already have the National Dex, which is given to you after you have seen every Pokemon in the region, and talk to the Professor.