A rating of a golf club shaft's ability to bend during the golf swing. All shafts, no matter how stiff, exhibit flex under the forces of the golf swing. A player with a very fast swing will require a shaft with less flex, while a player with a slower swing will need a shaft with greater flex. Flex is generally rated as Extra Stiff, Stiff, Regular, Senior and Ladies.
For in-depth information about shaft flex and golf shafts in general, see:
Also Known As: Shaft flex