noun, (tech)
A radio or radar set that upon receiving a designated signal emits a radio signal of its own and that is used especially for the detection, identification and location of objects.
This is the definition.
There are two different types of contactless RFID / transponders (RFID = Radio Frequency IDentification) :
  • passive transponders
  • active transponders
Compared with active transponders passive transponders do not need an own power supply. This leads to quite a range of features:
  • absolutely maintenance free
  • long life time
  • small dimensions
Passive RFID / transponders consist of a silicon chip and a coupling coil. The data communication and necessary energy for operation is transferred from the reader device to the transponder by the RF field. 
Goods or persons can be easily equipped with an electronically readable media. This is the reason for the alias "Tag".
We can supply RFID / transponders as:
  • plastic cards
  • coins
  • key fobs
  • glass tags
  • wristbands
By obtaining computer readable data of persons or goods in a contactless way, a lot of new applications are made possible or are simplifying industrial applications.

Typical applications:

Document identification
Documents, books or ID cards can be tagged with RFID / transponders which are brought onto thin self-adhesive foils. When trying to seperate document and transponder, both the document and the transponder will be destroyed.
Depending on the transponder type, there will be just a read/only identification number or the opportunity to write additional data. In case of document identification, it is then possible to track the document processing.
The unique serial numbers of read/only transponders can be used to identify persons or goods. 
Access control
The same RFID / transponders which are used for identification of persons can be used for access control systems. In this case, the unique serial number of a transponder is compared against a data base of allowed serial numbers. If it matches, the access is granted. The data base may be local in the access control terminal or remote at common data base server.
Public busses, ski lifts, motorway fees
RFID / Transponders featuring encryption and/or counter functions can be used as bus tickets, ski lift tickets and as media for toll stations collecting motorway fees. When entering a bus or motorway section or using a lift, a counter inside the RFID chip is decremented. If there are not enough counts left, the transponder can be "recharged" at assigned sales points.