A Proximity card is a "contactless" smart card that can be read without inserting a physical card into a reader device. Proximity cards, tags, fobs, or wristbands are all a type of "contactless" devices. When held near a proximity reader, a programmed card number is identified, and the reader produces a sound, or red/green light is activated to indicate the card is read.
The term "Proximity" or prox refers to a card or device that reads a frequency of 125 kHz. Newer contactless devices use a frequency of 13.56 MHz.