Ad-Hoc-Networks use point-to-point connections, i.e. the connection is always 1:1. No routers etc. involved. Do not confuse this tag with 'wifi-direct'. See full tag wiki for more details.
Usually this is a sub-class of the more generic wi-fi (see the Wikipedia article on Wireless ad hoc network). This type of connection is rarely supported by the Android system itself, but usually requires your device to be rooted and using special software (apps).
Other related tags include:
- wifi-direct: A new standard supported natively by newer Android versions, allowing peer-to-peer WiFi connections without the need of a router
- wifi-hotspot: Transforming your Android device into a wireless router serving up to 5 clients
- wifi-tethering: Can mean both, hotspot or peer-to-peer
- bluetooth-tethering: Same thing, just using Bluetooth instead of WiFi
- tether: The more generic term, which also can be applied to wired connections
- usb-tethering: Wired thethering using the USB connection
- reverse-tether: The opposite direction (using another device's network connection with your Android device)