rt.representation theory - What is Extreme/Extremal vector according to some weights

I know this might be a very elementary question. But I could not find the original definition of Extreme(or Extremal)vectors of some weights $lambda$(fixed the $win W$,where $W$ is Weyl group)

I am looking for definition for Extreme vector for finite dimensional Lie algebra and Affine Lie algebra. I found a paper saying :"Extreme vector verifies Weyl Character formula" What does it mean?

I am looking for reference talking about this concept. Thanks in advance

EDIT: I guess it is just highest weight vector, but I am not sure