NFC, or near field communication, is a very short range wireless standard that works over a few centimetres and requires a deliberate tap. It runs at 13.56 MHz and powers most contactless card and phone payments today.
The longer version
Near field communication is built for contact at a distance of a few centimetres. The short range is a feature. Because the two devices must almost touch, an NFC exchange is unlikely to happen by accident, which is part of why it became the standard for tap to pay cards and phones. The NFC Forum maintains the specification, which operates at 13.56 MHz.
That same short range is also a limit. NFC needs a deliberate, aimed gesture. The payer has to bring the device to the reader, which rules out paying while a phone stays in a pocket or across a counter. NFC is best understood as a special short range case of RFID, tuned for close, intentional contact.
In a proximity setting NFC is one available signal for confirming presence, suited to cases where a close, intentional tap is acceptable. Where the goal is to sense nearness without a tap, a longer range signal such as Bluetooth Low Energy or ultra wideband is the better fit.
The Parousya method
NFC is one of the four short range signals named in the Parousya patent. The method can use it where a deliberate tap is welcome, and reach for Bluetooth, ultra wideband, or WiFi Direct where nearness should be sensed without one.