sea wasp

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several box jellyfish having potent toxins in their tentacles, especially Chironex fleckeri of Australia, which can deliver a fatal sting.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A deadly marine stinging box jellyfish found in north east Australia and South East Asia, especially Chironex fleckeri.