sprung: a nib that responds to pressure, but without creating a significant variation in stroke, many modern nibs declared flexible fall into this category.
Demi-flex (semi-flexible): a nib which responds to pressure with a significant variation in stroke, but which is normally used without significant variation.
Flexible: a nib which produces a variation in stroke even in normal writing, in response to small variations in pressure exerted on it.
wet noodle (super-flexible): an extremely flexible nib, which must be used with care even in normal writing, leading to very accentuated stroke variations at minimum pressure.