Plain Vanilla

What Does Plain Vanilla Mean?
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, which alters the components of a traditional financial instrument, resulting in a more complex security.

Investopedia explains Plain Vanilla
For example, a plain vanilla 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 that the option only becomes active once the underlying stock hits a set price point.