  Poynting vector as electromagnetic power flow has prevailed over one hundred
years in the community. However in this paper, it is shown from Maxwell
equations that the Poynting vector may not represent the electromagnetic power
flow for a plane wave in a non-dispersive, lossless, non-conducting,
anisotropic uniform medium; this important conclusion revises the conventional
understanding of Poynting vector. It is also shown that this conclusion is
clearly supported by Fermat's principle and special theory of relativity.
