An hybrid of whip and sword, with the added feature of being retractile. The snake sword is a fictional weapon that gives it's user the sharpness of the sword, flexibility and range of a whip and retractile mechanism that imitates the bite of a snake.
Such a weapon would be much more impractical and complicated than it appears, as it would need an impressive engineering setup to create a mechanism and spring capable of holding the blade in sword-form without violating basic structural integrity and capable of sending it out as a whip. Also the wielder would need extensive practice to use it, or else risk damaging themselves trying to hit anything.
In Shinobido Edit
It's the weapon of choice of Hebitonbo. Unlike most fictional examples, Hebitonbo's weapon is mounted in a gauntlet and limited to extending in a straight line, despite cuscenes showing that it has the flexibility of a whip. The sharp end is inconsistently portrayed as capable of clawing into enemies.