ignoratio elenchi
noun
ig·no·ra·tio elen·chi ˌig-nə-ˈrä-tē-ˌō-i-ˈleŋ-ˌkē 
: a fallacy in logic of supposing a point proved or disproved by an argument proving or disproving something not at issue
Word History
Etymology
Latin, literally, ignorance of proof
First Known Use
1588, in the meaning defined above