Definition of "drop-in" [drop-in]

• One who casually drops in, as to visit or obtain an appointment. (noun)
• An informal social event. (noun)
• Provided for short-term use: a drop-in center for runaways. (adjective)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Products like Virent's are known as "drop-in" fuels because they can be put into the nation's existing fueling infrastructure and even produced at existing refineries."
• "In fact, Chameleon is being referred to as a drop-in replacement for UIKit, and in many cases, that's all that needs to be changed in order for an iOS app to run on the Mac."
• "The year before she arrived, the school could have been described as a drop-in community center, with students doing as they pleased."