  A wide range of materials, like d-wave superconductors, graphene, and
topological insulators, share a fundamental similarity: their low-energy
fermionic excitations behave as massless Dirac particles rather than fermions
obeying the usual Schrodinger Hamiltonian. This emergent behavior of Dirac
fermions in condensed matter systems defines the unifying framework for a class
of materials we call "Dirac materials''. In order to establish this class of
materials, we illustrate how Dirac fermions emerge in multiple entirely
different condensed matter systems and we discuss how Dirac fermions have been
identified experimentally using electron spectroscopy techniques
(angle-resolved photoemission spectroscopy and scanning tunneling
spectroscopy). As a consequence of their common low-energy excitations, this
diverse set of materials shares a significant number of universal properties in
the low-energy (infrared) limit. We review these common properties including
nodal points in the excitation spectrum, density of states, specific heat,
transport, thermodynamic properties, impurity resonances, and magnetic field
responses, as well as discuss many-body interaction effects. We further review
how the emergence of Dirac excitations is controlled by specific symmetries of
the material, such as time-reversal, gauge, and spin-orbit symmetries, and how
by breaking these symmetries a finite Dirac mass is generated. We give examples
of how the interaction of Dirac fermions with their distinct real material
background leads to rich novel physics with common fingerprints such as the
suppression of back scattering and impurity-induced resonant states.
