Cupule: A cup-like sheath surrounding or enclosing certain fruits, e.g. the acorn of Quercus (Oaks).*
The acorns on this English oak (Quercus robur) are maturing. The ‘caps’ at the top of the nuts are cupules.
* Allaby, M. (2006). A dictionary of plant sciences (Revised ed.). Oxford: Oxford University Press.