The standard version of something, such as a bond or option.  A plain vanilla item would not have any special components, unlike an exotic item which is more complex and varied.  One example of this is a plain vanilla option.
Related information about plain vanilla:
- Plain Vanilla Definition | Investopedia
 The most basic or standard version of a financial instrument, usually options,   bonds, futures and swaps. Plain vanilla is the opposite of an exotic instrument, ...
 
- Plain vanilla - Wikipedia, the free encyclopedia
 Plain vanilla is an adjective describing the simplest version of something, without   any optional extras, by analogy with vanilla ice cream, the default flavour.
 
- plain vanilla - definition of plain vanilla by the Free Online Dictionary ...
 adj. Informal. Lacking adornments or special features; basic or ordinary: plain   vanilla stock options; a plain vanilla wardrobe. Want to thank TFD for its existence  ?
 
- Plain Vanilla Shell
 Unlimited free searches of PlainVanillaShell.com's top quality data on retailers   and demographics, in the web community built for retail real estate professionals.
 
- Plain-vanilla - Merriam-Webster Online
 Oct 31, 2011 ... lacking special features or qualities : basic · See plain–vanilla defined for English  -language learners ». First Known Use of PLAIN-VANILLA ...
 
- What is plain vanilla? definition and meaning
 Definition of plain vanilla: The standard version of something, such as a bond or   option. A plain vanilla item would not have any special components, unlike an ...
 
- Plain Vanilla Games
 Twilight QuizUp. 13th of November, 2012. Big News! We just released an official   Twilight QuizUp game, on both iOS and Android! Our first release for both ...
 
- Plain Vanilla Mom
 So I've been playing around with a setup I've seen all over Pinterest since last   Christmas. Convinced with my trusty 50mm lens I could pull this off myself and ...