Sporadic

The qualification sporadic, indicating that occurrences of some phenomenon are rare and not systematic, can be used for:

Mathematics

 * Sporadic group, any of a small number of finite groups that do not fit into any infinite family of groups

Medicine

 * Sporadic disease, any of several diseases

Science

 * Sporadic E propagation, an unusual form of radio propagation occasionally allowing ing long-distance communication
 * Sporadic meteor, not part of a meteor swarm
 * Sporadic permafrost, discontinuous permafrost that covers only a fraction of an area