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.
The closest real-life equivalents are the chain-whip and the indian urumi, both which usually lack the same cutting power or reach.
In Shinobido[]
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.