Passive RFID

Passive RFID is typically referring to Gen2 UHF RFID (Class 1 Generation 2) and indicates an RFID Tag that does not contain a battery. Passive RFID tags harvest energy from radio waves and are very low-cost.

It can also refer to LF RFID and HF RFID -- however, these technologies are becoming obsolete due to advances in Gen2 technologies.

Please see

Gen2 is passive RFID, RFID definition, RFID Reader - how it works