speech

講演(こうえん)	Any public speech or talk given to a general audience. Authors doing a reading, dudes addressing the Friars’ Club, the graduation speaker at your college, are all doing 講演.

講義(こうぎ)	A lecture to a private group, a lecture at college.

セミナー	A seminar.

演説(えんぜつ)	A speech, usually political.