What is the criteria to know whether a lens is thin or thick ? Suppose we have a lens of r1= 10 mm , r2= -10 mm and thickness is 5 mm. So, with this information what can we say about the lens whether it is thin or thick ?
According to Wikipedia
For a thin lens, [the thickness] is much smaller than one of the radii of curvature.
Here you have thickness 5 mm and radii of 10 mm. From my point of view, 50% is not "much smaller". I'd say that for something to be much smaller or larger than something else, it should at least be one order of magnitude smaller/larger, so a factor of ~10 would be needed.
Thin lens is abstract construction it is just a plane with optical power. Another way to imagine it is lens with curvatures but inside the lens the ray height does not change.
Thick lens is a real lens with physical thickness and curvature.