Used with masters of any profession. Teachers, scholars, etc. Can be added to the end of a name (usually the surname), or simply 'sensei' in replacement of any name at all. Not to be confused with someone who is of higher status than you in the school or workplace (-sempai).
Saitou-sensei, kono tegami o Eigo ni yakushite itadakitai desu. (Teacher/master, I would like you to translate this letter into English for me.