Ibsen doesn't leave us guessing about what the title Ghosts means. There are four big mentions of ghosts in the play: when Regina and Oswald are caught making out; in Mrs. Alving's long speech about ghosts as dead ideas; when Mrs. Alving effectively ends her communication with Pastor Manders; and when Oswald tells his mother to give up the idea of mother-child affection. As the haunted, troubled protagonist, Mrs. Alving is always the first to utter the word "ghosts." These references help show us how her thinking about the past changes throughout the play.
Mrs. Alving's long speech on ghosts says the most about what Ibsen means. Mrs. Alving muses,
"I almost think we are all of us ghosts, Pastor Manders. It is not only what we have inherited from our father and mother that "walks" in us. It is all sorts of dead ideas, and lifeless old beliefs, and so forth. They have no vitality, but they cling to us all the same, and we cannot shake them off." (2.85)
While Mrs. Alving first considers Captain Alving the ghostly burden she wants to get rid of, she begins to see that her own social restraints and obligations are haunting her. Shedding them becomes her action in the play.