Gandalf

Gandalf was a name used to refer to the Doctor. When the Master sent a message to Martha Jones via a television broadcast, he showed her the aged Tenth Doctor and sarcastically said "Say hello, Gandalf."

The Eleventh Doctor compared himself to the character, saying he was "the outer space Gandalf". (DW: Meanwhile in the TARDIS)

Behind the scenes
Obviously, in the real world, Gandalf is a character from The Lord of the Rings by J. R. R. Tolkien. Televised Doctor Who never explicitly makes this connection.