A container designed to keep food and/or drinks cool.
Usually rectangular shaped, size varies from a few litres in volume to 100 or so litres. The smallest will hold a six-pack of 375mL cans, the largest can hold a weeks worth of beer for the most avid of partygoers, and requires two people to carry.
The esky is contructed from materials that insulate its contents, to keep it cold. When the food or drink is placed in the esky, ice (or some other cold substance) is put in too, with the purpose of keeping the temperature inside the esky low.
The esky is one of those Australian icons, like backyard cricket or the hills hoist. An esky is not complete unless it is full of beer. This, of course says something else about Aussies - they like their beer cold. Not cool, or room temperature like the poms, but cold. Eskies are important to Australians because anything related to beer is important (of course).
"Pack the esky! Piss-up at Freddy's!!"
or
Holy shit! The esky's empty! EMERGENCY!!"