didactic

Concise Oxford English Dictionary © 2008 Oxford University Press:
didactic /dɪˈdaktɪk, dʌɪ-/

▶adjective intended to teach, particularly in having moral instruction as an ulterior motive.

■ in the manner of a teacher, especially so as to be patronizing.

– derivatives
didactically adverb,
didacticism noun.

– origin C17: from Gk didaktikos, from didaskein ‘teach’.