Rear Admiral

Rear admiral (also known as rear admiral upper half or two star admiral) is a rank of flag officer in naval services and space forces, most notably Earth Starfleet and its Federation counterpart. Rear admiral is the second most basic flag officer rank, a grade higher than a commodore or one star admiral and a grade below vice admiral and other admiral grades in most rank structures. This rank is roughly equivalent with the authority of a major general of military forces. 

In military parlance, is it acceptable and normal to refer to a rear admiral simply as "admiral" in most forms of address.

Starfleet
In the Starfleet uniform insignia used in the mid-23rd century, an upper half rear admiral wore a service uniform rank insignia of a thick sleeve stripe of gold with braided edge (a commodore's insignia) with another separate braid above. This was slightly modified in the 2270s to the form of three grouped rank braids (again like a commodore's), with one stripe above. 



In the Starfleet uniform insignia introduced in the mid-24th century, a rear admiral wore a triangular rank braid with one pip underneath as their service uniform rank insignia. 


 * The correlation between this second lowest admiral rank and the uniform insignia worn in TNG season 1 is because other admirals were shown wearing higher and lower rank insignia, and the single braid insignia was the most basic one shown.

In a redesign of the admiral's service uniforms of 2365 and 2366, a new insignia of two gold pips enclosed in a rectangular gold border was worn by rear admirals, showing the origin of the "two star" name. This insignia continued into later uniform designs of the 2370s. 


 * The Star Trek Encyclopedia shows the insignia in a small rectangular pin, while canon appearances depict a wider rectangular pin. It is possible the insignia changed over the years between the mid 2360s and the later 2360s when those references took place, respectively.