Stowaway

According to K9 Mark II, a stowaway was "one who [hid] in a ship to obtain free passage". (TV: State of Decay)

The Doctor was sometimes accused of being a stowaway — even though he technically wasn't — because of the TARDIS' ability to land on ships. For example, the Third Doctor and Jo Grant were falsely accused of stowing away on the SS Bernice. (TV: Carnival of Monsters) Likewise, the Eleventh Doctor, Rory and Amy were briefly considered stowaways on the Fancy. (TV: The Curse of the Black Spot) But the Doctor also occasionally got called a stoway on spacefaring vessels, too. Once, the Tenth Doctor became a "stowaway" onboard the Titanic after the TARDIS collided with it. (TV: Voyage of the Damned)

Companions sometimes genuinely stole passage on the TARDIS, beginning their travels with the Doctor somewhat furtively. Steven, Zoe, Sarah, Leela, and Adric — to name but a few — were all stowaways. (TV: The Time Meddler, The Wheel in Space, The Time Warrior, The Face of Evil, State of Decay)