The Full Form of EIRP is‍ Equivalent Isotropically Radiated Power.

Effective, or Equivalent, Isotropically Radiated Power (EIRP) is the maximum amount of power that could be radiated from an antenna, given its antenna gain and the transmitter power of the RF system. EIRP is most commonly given in decibels over isotropic, dBi.

The IEEE definition for effective radiated power (ERP), which is similar to EIRP, is used to measure RF frequency sources, such as transmitters, and indicate the power of the mainlobe of the antenna that the system would radiate transmitting over a half-wave dipole antenna. The ERP measurement is equivalent to the input power of the antenna multiplied by the antenna gain. EIRP and ERP are different, in that EIRP is based on a hypothetical isotropic antenna, for which a half-wave dipole antenna has an antenna gain 1.64 times, or 2.15 decibels, that of an isotropic antenna.