The term "plain vanilla" is an idiomatic expression that refers to something that is simple, unadorned or basic. It describes a situation or thing that lacks any extra features, embellishments or complexities and is simply straightforward in its design or function. The phrase originated from the fact that vanilla ice cream, which is considered plain by many people, has been around for centuries without much change to its original recipe. In essence, "plain vanilla" means something that is ordinary, unassuming, and not particularly exciting or innovative.