en·thu·si·ast 
(ĕn-thzē-ăst′)
(ĕn-thzē-ăst′)n.
1. One who is filled with enthusiasm; one who is ardently absorbed in an interest or pursuit: a baseball enthusiast.
2. A zealot; a fanatic.
[Greek enthousiastēs, possessed person, from enthousiazein, to be inspired; see ENTHUSIASM.]