Definition Plain Vanilla
This is the most basic or standard version of a financial instrument, generally options, bonds, futures and swaps. Plain vanilla is the opposite of an exotic instrument, which alters the components of a traditional financial instrument, resulting in a more complex security.
This option is the standard type of option, one with a simple expiration date and strike price and no additional features. With an exotic option, such as a knock-in option, an additional contingency is added so as to the option becomes active only once the underlying stock hits a set price point.
Reading Source: Investopedia