enig·ma 
/ɪˈnɪgmə/ noun
plural enig·mas /ɪˈnɪgmə/ noun [count] : someone or something that is difficult to understand or explain
▪ To his friends, he was always something of an enigma. ▪ one of the great enigmas of our time
▪ To his friends, he was always something of an enigma. ▪ one of the great enigmas of our time
