Y-axis - Vertical axis on a graph. 

Yardang - A rock that has developed a streamlined form because of wind erosion. The long axis of these features is aligned with the dominant wind direction.

Yazoo Tributary - A small tributary channel that is prevented from joining the main stream channel by the presence of levees. Yazoo tributaries tend to flow on the floodplain parallel to the main stream channel.

Younger-Dryas - A cold period during the generally mild Holocene Epoch that occurred from about 10,000 to 8,500 BCE. Scientists speculate that this cooling may have been caused by the relatively rapid release of freshwater trapped behind ice on North America into the North Atlantic Ocean.