Nanno in cephalopod paleontology, as commonly used, refers to endocerids in which the siphuncle

Siphuncle

The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 swells to fill the entire apex of the shell leaving septa and camerae to begin a few centimeters forward, having a morphologic rather than a taxonomic connotation.

The term is based on the genus Nanno, named by Clarke in 1894 for the apical end of an endocerid from the Trenton Limestone of New York state that has the basic description, possibly the senior synonym for Proterovaginoceras

Proterovaginoceras

Proterovaginoceras is a medium to large sized endocerid from the Early and Middle Ordovician included in the family Endoceratidae....

. As a valid genus, Nanno is included in the Endoceratidae

Endoceratidae

The Endoceratidae is a family of large to very large straight shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician...

but the nanno type apex may be found in other families'