butter bean

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Chiefly Southern & Midland U.S. See lima bean.
2. n. New England See wax bean.

Wiktionary

1. n. Any of several beans, including the lima bean, sieva bean, wax bean, and civet bean.

WordNet 3.0

1. n. small flat green bean similar to lima beans
2. n. bush bean plant cultivated especially in southern United States having small flat edible seeds