Effective Isotropic Radiated Power
This is a way to
compare power output from
different antennas with different
gains and
directivity.
This is the
standard that the
FCC uses to
determine whether to
license transmissions over
airwaves for
Part 15 compliance.
For
example in the 2.4 GHz range of
802.11b networking you can have an
unlicensed antenna with 1
watt up to 6
dBi of gain for an EIRP of 4 watts but if you use an
isotropic antenna you can only output 1 watt (ie 1 watt EIRP).