vexillum

noun

vex·​il·​lum vek-ˈsi-ləm 

plural vexilla vek-ˈsi-lə 

1

: a square flag of the ancient Roman cavalry

2

: the web or vane of a feather

3

: the upper, large, often lobed petal of a papilionaceous flower (as of a pea or bean plant) : banner sense 6, standard sense 8a

The flowers are blue with a white patch on the vexillum (standard) which turns to purple as the flower ages …

—David Simons

compare keel entry 2 sense 2b, wing sense 2e(2)

vexillary

ˈvek-sə-ˌler-ē 

adjective

a vexillary corolla

Word History

Etymology

Latin; akin to Latin velum curtain, awning

First Known Use

1683, in the meaning defined at sense 1