Class T nebula

From Memory Alpha, the free Star Trek reference

A class T nebula is a type of small nebula, around a thousand kilometers in diameter, composed of hydrogen, helium, and argon.

The USS Voyager passed a T-class nebula in 2376. A Hierarchy assault starship was hidden inside the cloud. (VOY: "Tinker Tenor Doctor Spy")

A T-class nebula is located within three light years of the planet Dinaal. (VOY: "Critical Care")