Defence Against the Dark Arts professor from c. 1895 to c.1945. Soon after graduating from Hogwarts, Tom Riddle asked Headmaster Armando Dippet if he could remain and teach because Professor Merrythought was retiring after teaching for “nearly fifty years” (HBP20).
When Dumbledore would not allow Tom Riddle to become Defence Against the Dark Arts professor, he cursed the job and no one else could hold the position beyond one year. So until Voldemort’s final death, each professor who taught DADA left suddenly under unusual or disgraceful circumstances – or like Professor Quirrell, death (HBP20).
She must have been quite successful to stay in the job for fifty years.
Galatea is the name of a sea nymph connected in a love story to Polyphemus, the giant son of Poseidon the sea god.
It is also the name of the statue of a beautiful woman brought to life by a sculptor named Pygmalion in Ovid's "Metamorphosis."
"Merrythought" implies a happy frame of mind, which is useful for fighting against the Dark Arts