What is "pyrena"

Pyrena \Py*re"na\, n.; pl. Pyren[ae]. [NL., fr. Gr. ?, the stone of fruit.] (Bot.) A nutlet resembling a seed, or the kernel of a drupe.
--Gray.

n. (context botany English) A nutlet resembling a seed, or the kernel of a drupe

Pyrena or pyrene is the name for the stone within a drupe or drupelet. It consists of a seed surrounded by hard endocarp tissue.