Wiktionary
drop-down list
n. (alternative spelling of drop down list English)
Wikipedia
Drop-down list
A drop-down list (also drop down menu or drop-down menu or drop menu or pull-down list or simply a drop down) is a graphical control element, similar to a list box, that allows the user to choose one value from a list. When a drop-down list is inactive, it displays a single value. When activated, it displays (drops down) a list of values, from which the user may select one. When the user selects a new value, the control reverts to its inactive state, displaying the selected value. It is often used in the design of graphical user interfaces, including web design.