As I understand focal length it is the point inside the lens where the image crosses and becomes inverted, like the illustration in my avatar. (your view finder pentaprism flips the image back the right way)
This picture shows a pin hole camera and it happens at the pinhole, so the focal length is from the film plane to the pin hole.
The point where it happens on your 14mm lens is 14mm in front of the digital sensor and with your 24-70 zoom it moves as you zoom.
Most likely if you measure 14mm in front of the sensor you would reach a point in fresh air, the shape of the lens elements determines where that point will happen.
This is all a bit simplistic and I don't really understand much of the technicalities.
Focal length is a spec that comes with the lens and its only practical use is to associate it though familiarity of use with what it will look like though the view finder.
A better spec that does actually physically describe what we are interested in and directly relate what it will look like though the view finder is "Angle of view" but you won't find this printed on the lens, on the box it came in, or in general descriptions of a lens.
Because of this, if I said I had a 47° angle of view lens not many would relate to what that would look like though the view finder even though it describes it. If I said it was a 50mm lens (which has a 47° AOV) then though owning a 35mm film camera at some stage that came standard with a 50mm lens, then what that looks like though the view finder would be familiar.
If you put a 50mm lens on a crop frame camera then what that 50mm describes does not change, but what you see through the view finder does.
In the case of a Canon camera you get people describing it as an 80mm equivalent lens (50x 1.6 crop factor =80mm) when put on a crop frame camera, because the result though the view finder looks like a 80mm lens on a full frame camera.
If the spec "Angle of view" was the norm however the lens on a full frame would be a 47° AOV and on a crop frame it would be a 30° AOV.
So focal length does not mean a lot in practice unless you are a lens designer, its just the only spec per lens in wide circulation that can be made to tie in if only indirectly with what we expect to see when we choose to attach that lens over another in the camera bag.