A compound is a word containing a stem that is made up of more than one root.
(English)
Blackboard contains a stem that refers to "a large, smooth, usually dark surface on which to write or draw with chalk". However, the stem is made up of two roots, black and board.
Neufeldt, Virginia (editor). 1991.Webster's new world dictionary of American English. 3rd college edition. New York: Prentice Hall. 013949314X.