A space yacht was a type of spacecraft.
The Tiger Moth was a 24th century solar yacht. Karne stowed aboard trying to evade the Sontarans that were pursuing him. (HOMEVID: Shakedown: Return of the Sontarans, PROSE: Shakedown)
The Pinto was a space yacht on which the Seventh Doctor and Melanie Bush once battled the Quarks. (AUDIO: Flip-Flop)