n. A small ad hoc network created when two or more Bluetooth-compatible devices recognize and communicate with each other.
In normal operation, Bluetooth-enabled devices search for other units and configure themselves into small networks on an impromptu or ad-hoc basis. Two to eight Bluetooth units sharing the same channel form a piconet with one unit acting as the master.
—Warren Webb, “Bluetooth vendors bite the bullet,” EDN, March 29, 2001
Several piconets can communicate with each other simultaneously, creating a 'scatternet' that links, say, all the people around a conference table.
—“Is Bluetooth worth the wait?,” The Economist, December 09, 2000
1998 (earliest)
Bluetooth could end up competing with two other RF links for digital equipment, the HomeRF system and wireless LAN. All three work at the same frequency and have similar goals, although Bluetooth is designed to work anywhere, including away from the home or office. The systems should not interfere with each other because they use frequency hopping, spread spectrum techniques.

2.4GHz radio allows use worldwide

Spread spectrum frequency hopping (1,600hops/s)

10m personal 'bubble'

8 devices per 'piconet', 3 channels can be voice
—Tom Foremski, “Bluetooth bites into wireless,” Electronics Weekly, May 27, 1998
Bluetooth is a wireless networking standard that uses radio frequencies to set up a communications link between devices. The name comes from Harald Bluetooth, a 10th-century Danish king who united the provinces of Denmark under a single crown, the same way that, theoretically, Bluetooth will unite the world of portable, wireless devices under a single standard. Why name a modern technology after an obscure Danish king? Here's a clue: two of the most important companies backing the Bluetooth standard — Ericsson and Nokia — are Scandinavian.

Piconet combines the prefix pico-, "very small; one trillionth," with the noun network. And, just so you know, if you have a piconet operating at a particular frequency and one or more other piconets operating on different frequencies, they can communicate with each other, and the resulting network is called a scatternet (see the second citation).