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).