Wikipedia has a disambiguation page for this term. The sense that I am interested in is the mathematical one, that is, either half of a double cone. This term (for any of the senses) does not appear in Benson, nor in Wells, nor in Vikivortaro. Wikipedia mentions, and defines, this term in its article on ‘cone’, but does not have a separate article on it, and therefore if there is an article on it in Vikipedio it is not reachable from the sidebar of the English-language article. I went directly to Vikipedio and tried bringing up the term with a few guesses, such as ‘napo’, ‘napeo’, and ‘napio’, but to no avail. I did the same with the online PIV, again to no avail. So, how should the mathematical sense of ‘nappe’ be rendered in Esperanto?
Note that for mathematical terminology, ReVo is much more trustworthy than either PIV or Vikipedio, as it contains the full content of the Matematika vortaro kaj oklingva leksikono by Marc Bavant, the best source of mathematical terms in Esperanto. According to the definition in ReVo, konuso – when used to refer to a surface rather than a solid – actually means double cone (see definition a)), and the half of this surface that borders a solid cone is called duona konuso (see definition b)). Additionally, a comment after the definitions clarifies that one should use konusa surfaco or konusa solido in case there is a risk of misunderstanding about which meaning is meant. If I correctly understand the English nappe, it always refers to a surface, and never to a solid. So following this terminology, one should say either duona konuso or duonkonusa surfaco for a nappe.
The word nappe ("tablecloth") is just the French term for a sheet of a surface.
In English we normally say, for example, hyperboloid of one sheet rather than one nappe.
Because a double cone is connected at the vertex, there seems to be hesitation about whether it is in one or two sheets, and this may have led people to follow French books of geometry and say it is in two nappes. However, I think it is probably not necessary in context to use a separate word.
In its entry for hiperbolo, PIV gives the examples unupeca, dupeca hiperboloido.
So my guess is that you could say la supra, suba peco de duoblakonuso.
Obviously, tuko and folio might be used, but peco is what they chose.
In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi)... https://en.wikipedia.org/wiki/Nappe
I found this term tertavolo in ReVo, but I don't know.
tertavolo = Horizontala tavolo da tero