Gandalf

Gandalf was a name used to refer to the Doctor. When sent a message to Martha Jones via a television broadcast, he showed her the aged Tenth Doctor and sarcastically said, "Say hello, Gandalf." (TV: Last of the Time Lords)

The Eleventh Doctor compared himself to the character, saying he was "the outer space Gandalf", but incorrectly followed this by describing him as "the little green one in Star Wars." (TV: Meanwhile in the TARDIS)

Behind the scenes

 * In the real world, Gandalf is a character from The Lord of the Rings and The Hobbit by J. R. R. Tolkien. Televised Doctor Who never explicitly makes this connection.
 * Ian McKellen played Gandalf in Peter Jackson's The Lord of the Rings trilogy; he later provided his voice for the Great Intelligence in The Snowmen. He also made a cameo as himself in costume as Gandalf on the set of The Hobbit in The Five(ish) Doctors Reboot.
 * Fourth Doctor actor Tom Baker was briefly considered for the part of Gandalf in The Lord of the Rings film trilogy; coincidentally, Seventh Doctor Sylvester McCoy would later be cast as Gandalf's fellow wizard Radagast the Brown.
 * Gandalf appears as one of the main characters in LEGO Dimensions.