The biggest producer of hazelnuts, Corylus avellana, in the U.S. is Oregon, "In Oregon, you grow filberts, but you sell them as hazelnuts," says Anita Azarenko, an Oregon State University horticulture professor with an organic apple and hazelnut orchard.  The "hazel" comes from the Anglo-Saxon word for cap or hat; the tree's genus name Corylus also invokes the helmetlike shape of the nuts. Oregon growers (and their marketing board) officially started calling their product "hazelnuts" in the 1980s, as demand for exports grew. Befuddled customers had been sending their "filbert" shipments back.  An old-fashioned name, introduced by French settlers, for the same nuts, which the British, not to complicate things, call cobnuts. 

Hazelnuts are a staple in European confections and baked goods, as well as an ingredient in Fererro's popular Nutella spread. But hazelnuts have another popular name—filberts. How did that happen?

The most widely believed story explaining this second name is steeped in religion. The feast day of St. Philbert, a French saint, falls on August 20th. That also happens to be peak harvest time for hazelnuts, which traditionally mature in late August. So people started applying the saint's name to the nuts that were in season on his feast day. Hazelnuts have even more aliases in the US: some people call them cob nuts, and still others simply call them hazels.

More: http://www.hort.uconn.edu/plants/detail.php?pid=132

https://www.ncbi.nlm.nih.gov/pubmed/12797745

http://www.actahort.org/books/351/351_19.htm

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