A runlet is a small stream of water that flows with a gentle movement. There are several synonyms for the word runlet that you can use to describe this type of watercourse. These include streamlet, rivulet, rill, brook, and creek among others. Each of these words has a slightly different connotation, but they all refer to a small body of water that runs along a more or less defined path. Streamlet is the closest synonym to runlet, and it is often used interchangeably. Rill is used to describe a tiny and shallow stream. Brook and creek, on the other hand, imply a slightly larger size and can sometimes refer to permanent water sources.