vex·il·lum 
(vĕk-sĭləm)
(vĕk-sĭləm)n. pl. vex·il·la (vĕk-sĭlə)
1. A usually square banner hung from a crossbar on a staff, used as a military standard in ancient Rome.
2. Botany See standard.
3. Zoology The weblike part of a feather; the vane.
[Latin, banner; akin to vēlum, sail.]