  We propose the concept of fermion condensation in bosonic topological orders
in two spatial dimensions. Fermion condensation can be realized as gapped
domain walls between bosonic and fermionic topological orders, which are
thought of as a real-space phase transitions from bosonic to fermionic
topological orders. This generalizes the previous idea of understanding boson
condensation as gapped domain walls between bosonic topological orders. We show
that generic fermion condensation obeys a Hierarchy Principle by which it can
be decomposed into a boson condensation followed by a minimal fermion
condensation, which involves a single self-fermion that is its own
anti-particle and has unit quantum dimension. We then develop the rules of
minimal fermion condensation, which together with the known rules of boson
condensation, provides a full set of rules of fermion condensation. Our studies
point to an exact mapping between the Hilbert spaces of a bosonic topological
order and a fermionic topological order that share a gapped domain wall.
