Plain-language answer: EIRP is transmitter power after feed-system loss, multiplied by antenna gain referenced to an isotropic radiator.
Why the definition matters
EIRP and ERP describe the same physical chain with different antenna references; EIRP is about 2.15 dB higher than ERP for the same system and direction.
Use the number correctly
Calculate at the specified frequency and direction with actual feed losses and dBi gain, then apply current regulatory requirements.
The common mistake
Adding transmitter watts directly to antenna dBi.
Measurement and safety boundary
Keep reference units, bandwidth, detector, direction, averaging, and test conditions attached to every number. Current regulator requirements and the complete installed system control RF-exposure compliance.
