noun pl. Hydras -·dras or Hydrae -·drae′ (-drē′)

1. Gr. Myth. the nine-headed serpent slain by Hercules as one of his twelve labors: when any one of its heads is cut off, it is replaced by two others
2. a long S constellation between Cancer and Libra: the largest constellation
3. any persistent or ever-increasing evil with many sources and causes
4. any of a family (Hydridae) of small, freshwater, solitary hydroids having a dominant soft-bodied polyp stage

Etymology: ME ydre (< OFr < L), ydra < L Hydra < Gr, water serpent, akin to hydōr, water