Reyamilk is a liquid substance used on Risa for relaxation.
The Federation Travel Guide recommended a relaxing reyamilk soak while one was a tourist was on Risa. (ST reference: Federation Travel Guide)
In 2373, Leeta planned to take a reyamilk soak with a young Risian. She later asked Jadzia Dax and Worf to join them. (DS9 episode: "Let He Who Is Without Sin...")