According to K9 Mark II, a stowaway was "one who [hid] in a ship to obtain free passage". (TV: State of Decay) The Sixth Doctor told H.G. Wells, who had stowed aboard his TARDIS, that there was an intergalactic law expressly forbidding stowaways. (TV: Timelash)
The Doctor and their companions were sometimes accused of being stowaways — even though they technically weren'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 stowaway 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) Similarly, the Sixth Doctor and Evelyn Smythe were thought to have stowed away on the Valiant when they had really just materialised there minutes before. (AUDIO: The Nowhere Place)
Companions sometimes genuinely stole passage on the TARDIS, beginning their travels with the Doctor somewhat furtively. Steven Taylor, (TV: "The Watcher") Zoe Heriot, (TV: The Wheel in Space) Sarah Jane Smith, (TV: The Time Warrior) Leela (TV: The Face of Evil) and Adric (TV: State of Decay) — to name but a few — were all stowaways.