Let X be a variety over F_p . Fix a prime l different from p, an algebraic closure \overline{Q_l} of Q_l, and a p-adic valuation v of the subfield of algebraic numbers in \oversline{Q_l} normalized so that v(p)=1. Let M be a \overline{Q_l}-sheaf on X such that for every closed point x in X the eigenvalues of the geometric Frobenius acting on M_x are algebraic numbers. Applying the valuation v to these numbers and dividing by the degree of x over F_p, we get a collection of rational numbers, which are called slopes of M at x. Read more about Special Wolf Prize lecture: Vladimir Drinfeld (Chicago): Slopes of irreducible local systems
