Spiral or helical stair

A true spiral stair generally will have a central column that the structure radiates from, whereas a helical shape is free form and typically has a balustrade to both sides. Spiral is still the common term to this style of staircase. Often used as a space saving alternative, a helical or spiral stair design will require detailed design in reference to the NCC to ensure compliance. 

A double helix is when there are 2 helical stairs servicing the same zones and are intertwined over one another, such as in the UTS Central staircase.