What characteristic defines a passive RFID tag?

A passive RFID tag is characterized by not having its own power source and remaining dormant until activated by an external energy source, typically from an RFID reader. When the reader emits radio waves, the tag captures this energy and uses it to power its circuits briefly, allowing it to transmit data back to the reader. This design enables passive RFID tags to be smaller, lighter, and more cost-effective than their active counterparts, which contain their own battery and are capable of continuous data transmission.

In contrast, active RFID tags are always on and continuously transmit data because they have an internal power source. The size and battery requirements also differentiate them from passive tags—passive tags generally do not require frequent battery replacements, as they do not have batteries to begin with. Furthermore, passive tags are usually smaller than active tags, which often need to accommodate the battery. This unique operational principle of passive RFID tags is what distinguishes them from other types.