  We provide classification of gapless phases in non-Hermitian systems
according to two types of complex-energy gaps: point gap and line gap. We show
that exceptional points, at which not only eigenenergies but also eigenstates
coalesce, are characterized by gap closing of point gaps with nontrivial
topological charges. Moreover, we find that bulk Fermi arcs accompanying
exceptional points are robust because of topological charges for real line
gaps. On the basis of the classification, some examples are also discussed.
