"Don" wrote
A spiral stair is not shaped like a cone.

"CW" wrote
It's not named correctly either.

"Don" wrote
Why?

"CW" wrote
Because it is not a spiral. I just looked it up and, like a lot of words
that are commonly misused, the definition of spiral has been *******ized.
In
my line of work, where geometric correctness is important, no one would
ever
interchange the terms.

From www.m-w.com is this entry for the NOUN spiral:

Main Entry: 2spiral
Function: noun
1 a : the path of a point in a plane moving around a central point while
continuously receding from or approaching it b : a three-dimensional curve
(as a helix) with one or more turns about an axis
2 : a single turn or coil in a spiral object
3 a : something having a spiral form b (1) : a spiral flight (2) : a kick or
pass in which a football rotates on its long axis while moving through the
air
4 : a continuously spreading and accelerating increase or decrease wage
spirals

Now please explain how #1a above does NOT describe a spiral stair.
Notice how the word *cone* is not mentioned at all.

Here's the www.m-w.com entry for helix:

Main Entry: he·lix
Pronunciation: 'hE-liks
Function: noun
Inflected Form(s): plural he·li·ces /'he-l&-"sEz, 'hE-/; also he·lix·es
/'hE-lik-s&z/
Etymology: Latin, from Greek; akin to Greek eilyein to roll, wrap -- more at
VOLUBLE
1 : something spiral in form: as a : an ornamental volute b : a coil formed
by winding wire around a uniform tube
2 : the incurved rim of the external ear
3 : a curve traced on a cylinder or cone by the rotation of a point crossing
its right sections at a constant oblique angle; broadly :

I believe the term *spiral stair* is correct.