Nebula

Nebula can have several meanings:


 * Advanced Research Vessel or Advanced Research Vessel Retrofit, of which the "Nebula Class" is one variant.

or


 * An astronomical object, largely consisting of gas and dust. In Star Trek Online they may either be part of a sector, e.g., the Mutara Nebula, or are an exploration cluster of their own, e.g., the Betreka Nebula.