Air Lock

From Distillers Wiki

A glass or plastic device designed to use water as an insulator to protect the fermentation media from contamination and exposure to fresh air, while at the same time allowing carbon dioxide produced by the yeast to escape the fermentation vessel. Also called a fermentation trap, bubbler or airlock. Perhaps the simplest airlock often used by amateur brewers is a common party balloon that has a pinprick hole to allow CO₂ to escape.