Flex or flexible nibs are fountain pen nibs which produce a line which varies in width with the pressure used. A very flexible nib can produce a width variation of about six times. Extremely flexible nibs are sometimes known among collectors as "wet noodles".
Flex nibs should not be confused with nibs that merely offer a cushioned feel in writing, or even those that vary line wetness and colour saturation with pressure. Such nibs are known merely as 'soft'.