”Butter bean” is a term that some use for particular types of lima beans; the only difference is in regional terminology. The lima bean is a domesticated legume with the scientific name of Phaseolus lunatus that gets its name from its place of origin, Lima, Peru. Some in the American South and the United Kingdom call particular types of lima beans “butter beans.”
Dried lima beans that are more flat and yellowish than the common rounded and green variety are often called “butter beans” in Southern States. In the South, lima beans and butter beans are considered to be two different types of bean, however, in their scientific name is the same and they are not two distinct species of bean.
The culinary terminology used in the United Kingdom differentiates between lima and butter beans more in line with the Southern United States distinction, where lima beans that are dried, larger, and light yellow are referred to as butter beans and the smaller green bean is called a “baby” or “junior” lima bean. These distinctions do not change the scientific terminology, and the two common terms used for the lima bean are often interchangeable regardless of regional or culinary preferences.