The transparent, drifting body form of a jellyfish is called a medusa.
In zoology, medusa describes the bell-shaped, free-swimming stage found in many cnidarians, especially true jellyfish. Its bell contracts to propel the animal through the water, while tentacles and oral arms help capture prey and move food toward the mouth.
Many cnidarians have a life cycle that alternates between a medusa stage and a polyp stage. Polyps are generally attached to a surface and have a tube-like body, whereas medusae are usually mobile and reproduce sexually. The details differ among groups, so not every cnidarian follows an identical cycle.
A planula is a ciliated larva, not the adult drifting form. An ephyra is a juvenile stage in some true jellyfish, produced before it develops into a mature medusa. These distinctions explain why several jellyfish life-cycle terms are easy to mix up.