Of a computer menu, opening downward to reveal a list of possible options.(adjective)
A type of menu that, when selected, opens downward to reveal a list of possible options.(noun)
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Use "drop-down" in a sentence
"Twitter also will offer filtering options, such as searching for tweets based on your location, and saved searches will appear in a more noticeable drop-down menu."
"For those who are wondering where they can locate the zoom feature, on Internet Explorer, it's located under the "Page" drop-down menu, to the right of the little "home page" and printer icons just above the viewable window where you view whatever Web site you're on."
"Notifications and common settings like Wi-Fi connections are handled discreetly and easily accessed via drop-down icons at the top of the screen."