Wiktionary
hafnian
a. (context mineralogy English) Describing minerals containing hafnium
Wikipedia
Hafnian
In mathematics, the hafnian of an adjacency matrix of a graph is the number of perfect matchings in the graph. It was so named by Eduardo R. Caianiello "to mark the fruitful period of stay in Copenhagen (Hafnia in Latin)."
The hafnian is computed as
\operatorname{haf}(A) = \frac{1}{m!2^m} \sum_{\sigma \in S_n} \prod_{j=1}^m A_{\sigma(2j - 1), \sigma(2j)},
where S is the symmetric group on [n].