Ethylene: A gas (C2H4) that is produced naturally by plants, and which functions as a hormone in the control of such processes as germination, cell growth, fruit ripening, senescence, and abscission. It is also involved in the response of a plant to gravity and to stress.*
Some fruits, such as apples and bananas, give off a lot of ethylene gas as they ripen. This is why placing them in a bag makes them ripen faster, as the gas is trapped and concentrates around the fruit. If you have fresh flowers in the kitchen, be sure to keep them away from your fruit bowl as ethylene given off by the fruit can cause your flowers to age faster!
*Allaby, M. (2006). A dictionary of plant sciences (Revised ed.). Oxford: Oxford University Press.