The Bay Harbor Butcher was the name the press gave to the perpetrator of Dexter Morgan's mass murder of Miami's serial killers. The name derives from Dexter's method of disposing of the remains of his victims, in which they were chopped up and sunk to the bottom of the bay. Dexter himself expressed disdain for the name.
When it became public knowledge that the Bay Harbor Butcher only killed other killers, the public began expressing affection for the figure. This led to the creation of the Dark Defender, a comic book antihero concept based on the Bay Harbor Butcher.
Dexter was ultimately able to frame James Doakes as the Bay Harbor Butcher. Five years later, Captian Maria LaGuerta finds a bloodslide exactly like the kind the Bay Harbor Butcher takes. After this, she looks into Dexter's past and suspected Dexter as The Bay Harbor Butcher. However, at the end of season 7, Maria is killed by Debra Morgan after learning the truth that Dexter Morgan is The Bay Harbor Butcher.