Armando Dippet is the Hogwarts headmaster before Albus Dumbledore in J.K. Rowling’s history. Dippet is shown in a portrait and discussed in the Harry Potter books, especially in connection with Tom Riddle’s time at Hogwarts.
Dippet was headmaster when Riddle attended the school in the 1940s. In Harry Potter and the Chamber of Secrets, Riddle’s memory reveals that he asked Dippet for permission to remain at Hogwarts during the summer because he had nowhere else to go. Dippet refused, partly because the school was considered unsafe.
Dumbledore later became headmaster and remained one of Hogwarts’ most influential leaders. His earlier career included teaching Transfiguration at Hogwarts, and he was already a respected wizard when he succeeded Dippet.
Dippet should not be confused with Phineas Nigellus Black, who was also a former Hogwarts headmaster, or with Horace Slughorn, who served as a professor and Head of Slytherin. The chronology is clearest from the school memories and historical references surrounding young Tom Riddle.