Definition of "add-on" []

• One thing added as a supplement to another. (noun)
• Computer Science See add-in. (noun)
• Computer Science A hardware device, such as an expansion card, that is added to a computer to increase its capabilities. (noun)
• An additional amount or charge. (noun)

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.

• "Travelers who choose an add-on, which is tacked on to existing domestic calling and data plans, now get 50 MB for $25 a month compared with 20 MB previously, and up to 800 MB for $200 a month versus 200 MB."
• "We are preparing a system for micro-transactions and so-called add-on content for both the Nintendo 3DS and the Wii U, and for the Nintendo 3DS we are targeting to release something this calendar year."
• "The upgrade costs for NFC range from $100 to $850 per terminal, compared with an add-on device to support Zoosh that will cost merchants about $30, Naratte estimates."