A small device, designed to be affixed to the top of a fermenter. An airlock allows the CO2 produced during fermentation to escape, without allowing airborne bacteria to enter the fermenter. The most common type is called a three piece airlock, and consists of a small plastic chamber, a float which sits inside the chamber, and a dust cover. The chamber is partially filled with liquid - plain water, sanitizing solution, or vodka, depending on how paranoid you are - and the CO2 bubbles out from underneath the float. There is also a single-piece, "sideways S" type of airlock which is in fairly widespread use.