  ``EIT waves" are large-scale coronal bright fronts (CBFs) that were first
observed in 195 \AA\ images obtained using the Extreme-ultraviolet Imaging
Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}.
Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that
propagate pseudo-radially across the solar disk at velocities of 100--700 km
s$^{-1}$ with front widths of 50-100 Mm. As their speed is greater than the
quiet coronal sound speed ($c_s\leq$200 km s$^{-1}$) and comparable to the
local Alfv\'{e}n speed ($v_A\leq$1000 km s$^{-1}$), they were initially
interpreted as fast-mode magnetoacoustic waves ($v_{f}=(c_s^2 + v_A^2)^{1/2}$).
Their propagation is now known to be modified by regions where the magnetosonic
sound speed varies, such as active regions and coronal holes, but there is also
evidence for stationary CBFs at coronal hole boundaries. The latter has led to
the suggestion that they may be a manifestation of a processes such as Joule
heating or magnetic reconnection, rather than a wave-related phenomena. While
the general morphological and kinematic properties of CBFs and their
association with coronal mass ejections have now been well described, there are
many questions regarding their excitation and propagation. In particular, the
theoretical interpretation of these enigmatic events as magnetohydrodynamic
waves or due to changes in magnetic topology remains the topic of much debate.

Solar and Stellar Astrophysics