(1) See USB drive.
(2) An adapter of some type that plugs into the USB port, including a mouse/keyboard transreceiver, 3G/4G cellular adapter or security device. Including the plug, USB dongles are typically from one to three inches in length. The word dongle was coined years ago when software copyright protection was built into USB hardware keys that plugged into each other. When several of them were used, they "dangled" off the back of the computer (see hardware key). See wireless adapter.