Wiktionary
hex key
n. A hex head wrench.
Wikipedia
Hex key
A hex key, Allen key or Allen wrench is a tool of hexagonal cross-section used to drive bolts and screws that have a hexagonal socket in the head (internal-wrenching hexagon drive).
The Allen name is a registered trademark, originated by the Allen Manufacturing Company of Hartford, Connecticut circa 1910, and currently owned by Apex Tool Group, LLC. Its genericised use is discouraged by this company. The standard generic name used in catalogues and published books and journals is "hex key".