standard petal; banner; vexillum
Botanical term for the large, often upright petal in papilionaceous flowers (e.g., peas, beans). Also called a banner or vexillum.
エンドウの花の旗弁は大きくて目立つ。
The standard petal of a pea flower is large and conspicuous.
Compound of 旗 (flag, banner) and 弁 (petal), referring to the petal's banner-like appearance. The exact historical derivation is uncertain; the spelling is conventionally associated with this botanical meaning.