Let $G$ be a reductive group over a field $k$ of characteristic zero with maximal split torus $T$ of rank $n$ and a Borel $B \supset T$ defining a set of simple roots $\Delta$. By $X_*(T)$ we denote the cocharacter group.
I read that there is a dominance order on $X_*(T)$ with respect to $B$ but couldn't find any formal definition. As the dominance order is mentioned with respect to $B$, I guess that the simple roots are involved too.