diesis
noun
di·esis ˈdī-ə-səs 
plural dieses ˈdī-ə-ˌsēz
: double dagger
Word History
Etymology
probably from Italian, sharp (in music), symbol for a sharp, from Medieval Latin, quarter tone, from Latin, from Greek, from diiēnai to send through, from dia- + hienai to send — more at jet
First Known Use
circa 1706, in the meaning defined above