An explorer was someone who went to places about which little was known to learn more about them.
Ulysses was a Time Lord explorer. (PROSE: The Gallifrey Chronicles)
The First Doctor, Ian, Barbara and Susan encountered Venetian trader and explorer Marco Polo on the Plain of Pamir in 1289. (TV: Marco Polo)
Professor Edward Travers helped the Second Doctor defeat the Great Intelligence. (TV: The Abominable Snowmen)
Ernest Shackleton met the Tenth Doctor and Martha Jones on an expedition in Antarctica, where they encountered the Skith. (COMIC: The First)
The Eleventh Doctor recruited Edwardian explorer John Riddell from 1902 to answer a 24th century distress call. (TV: Dinosaurs on a Spaceship)
Shalabar Scone was a legendary explorer on Lahn. (COMIC: The Spice Route)