What does one-dimension mean for thermal conductivity? I am starting to learn the Fourier's law, but I am getting confused with the following sentence on Wikipedia:

The above differential equation, when integrated for a homogeneous material of 1-D geometry between two endpoints at constant temperature, gives the heat flow rate as:

I am wondering what "1-D geometry" means (isn't all the thing we deal with 3D geometry?) and curious what we could use this formula for (which shape?):
$$\frac{Q}{\Delta t} = -kA\frac{\Delta T}{\Delta x}$$
Also, could I use this formula for determining the heat transfer for a bottle that looks something like this?

If not, what should be the equation I use for the heat transfer for that bottle? I tried to search for the three dimensional form of Fourier's law, but found limited results and no useful information.
 A: One dimension here means that the problem is homogeneous in the dimensions perpendicular to the dimension of interest. This means that the equations to be analyzed depend only on one variable, i.e., they are one-dimensional equations.
This is different from one-dimensional as understood, e.g., when applied to a thin rod, or a pipe (or one-dimensional structures on nanoscale, such as quantum wires) - where we would be talking about a system that is physically one-dimensional, in the sense that its length is much bigger than its width (transversal size.)
A: 
what does "1 D geometry" means (isn't all the thing we deal with 3D geometry?)

Yes, our world is 3D. But if you had a very long item, insulated along the sides, the temperature variation throughout the cross-section might be minimal and uninteresting compared to the temperature variation along the length. Thus, you might model the heat flux as $q=-k\frac{dT}{dx}$ for simplicity.
More generally, you’d model the heat flux as $q=-k\nabla T$, the 3D version.
