whip sword
A combination sword and whip. Mostly associated with Ivy from the Soulcalibur series, Signum's Levantine from the Mahou Shoujo Lyrical Nanoha series and Galient's sword from the Kikou-Kai Galient. Also, do not confuse this tag with chainsword, which is a sword with moving chainsaw parts.
Pixiv tags are 蛇腹剣 (lit. accordion sword) and ガリアンソード (Galient's sword).