1. One of the zooids of certain Siphonophora having the relative shape and the essential structure of the bell of a jellyfish: The nectocalyx uses its form and body structure as a swimming organ.
2. The swimming bell or umbrella of a jellyfish of medusa; nectophore: When Sammy was standing close to the side of the harbor, he could see the nectocalyx of the Portuguese man-of-war floating close to the surface.
3. A modified medusiform adapted for swimming purposes found as part of a siphonophore colony: In the science book James was reading, a nectocalyx was described in regards to its movements in water.