if sin(radian) is for x,
and cos(radian) is for y,
what is for z?
this might noght be descriptive enough, so feel free to ask for clarification
You can think of the plane as the "top" of a cylinder. The important things on the top can be defined by co-ordinates, or coordinates, where and .
The height of the cylinder is defined by the length on the axis in space. Since this doesn't depend on or , it doesn't depend on or either. Therefore, is just .
So to answer your question
These are known as Cylindrical Polar Co-ordinates.
Or it could be the Spherical coordinate system, which if you draw a line 1 unit and move it about the origin, then find where the furthest point is from the origin.
If this is the case, then x is no longer just cosine and y is no longer just sine, because as you move it about (0,0,0) through the z-axis as well, then it will be shorter than just sin(x) and cos(x).