An Orion vessel was a type of vessel used by Orion pirates, a design that had not yet been encountered by the Federation in 2270. Orion vessels were armed with phasers (albeit not very strong ones) and emit a unique trail of radioactive waste that is visible on a spectral analysis.
In 2270, an Orion vessel attacked the USS Huron for its cargo of dilithium crystals, as well as a special supply of strobolin. It was later encountered by the USS Enterprise in a field of asteroids, where it was eventually captured. (TAS: "The Pirates of Orion")