Mary Riddle (1883-1943) was the wife of Thomas Riddle and mother of Tom Riddle and thus the paternal grandmother of Tom Marvolo Riddle. She was part of the wealthiest family in the village of Little Hangleton. The Riddles were snobby and looked down on most other people in town, in particular the poverty-stricken House of Gaunt that lived nearby, becoming very unpopular amongst them.
Mary and her husband suffered great scandal when Tom abruptly married Merope Gaunt, the daughter of "tramp" Marvolo Gaunt. Merope was one of the Gaunts, a wizarding family. She had presumably entranced Tom by means of the Imperius Curse or, more likely, a Love Potion. The couple left for London, where Merope became pregnant. Abandoning his pregnant wife, Tom soon returned to his parents' home in Little Hangleton, claiming to have been "hoodwinked" and "taken in" by Merope. Merope would die soon after giving birth to Tom Marvolo Riddle, living only long enough to name him.
In 1943, sixteen-year-old Tom Marvolo Riddle, who would later take on the alias Lord Voldemort, ventured to Little Hangleton to investigate his heritage. He learned the details of his parents' union from his uncle Morfin Gaunt (Merope's brother) and sought revenge on what he saw as the man who caused his mother's death. Tom Marvolo Riddle went to the Riddle House to murder his father and ended up killing Mary and Thomas Riddle in order to erase his "filthy" Muggle heritance, too. Mary was buried in the Little Hangleton graveyard along with her son and husband, but their deaths were not mourned by the town due to their snobbishness.
The name "Mary" is the usual English form of Maria, which was the Latin form of the New Testament Greek names Μαριαμ (Mariam) and Μαρια (Maria) - the spellings are interchangeable - which were from the Hebrew name מִרְיָם (Miryam). The meaning is not known for certain, but there are several theories including "sea of bitterness", "rebelliousness", and "wished for child". However it was most likely originally an Egyptian name, perhaps derived in part from mry "beloved" or mr "love".