The story
So called because they are related to flags of modules.
Etymology from Wiktionary, CC BY-SA 4.0.
vexillary permutation
Today it means: A permutation μ of the positive integers containing no subpermutation isomorphic to the permutation (2143); in other words, there do not exist four numbers i < j < k < l with μ(j) < μ(i) < μ(l) < μ(k). Full definition →