The Thasian ship was a space-faring vessel utilized by the non-corporeal Thasians. Apparently energy-based, the ship took on the appearance of an indistinct, green mass of light.
In 2266, this ship intercepted the starship USS Enterprise to retrieve Charlie Evans, a young man whom the Thasians had given psychokinetic abilities which they themselves also possessed. The Thasian ship was scheduled to take him back to their homeworld of Thasus, due to Charlie and his powers having proved dangerous to others. (TOS: "Charlie X")
In the final draft script of "Charlie X", the Thasian ship was described as "a transparent blob [...] ... Ectoplasmic.