When do you use the expression "or as"? and what does it mean?
You use the expression "or as" when you are linking alternatives together. It's a piece of alternative information to give the listener more context to the subject. Dumbledore is informing Harry that he has met Voldemort back when he was known as Tom Riddle; that Tom didn't go by the name Voldemort back then. He is linking Voldemort's current title to his birth title. Ex: My mother's name is Mary Smith or as she is more commonly called Mama Mary. Ex: Lilies, or as they are scientifically named Lilium, are very beautiful. Ex: Tea, or as I like to call it, leaf juice, is my favorite drink.