What is ouji?
Ouji/ouji-sama (王子), literally "prince," is the  style that is often associated with 
lolita fashion. Even if the style is more masculine, it can be worn by both males and females. 
It has a variety of looks often combining pirate, punk, cute, or gothic elements. Other an also be used to describe this style, but they are not used as often  as ouji.
What not to call it
 Often it can be mistaken for Gothic aristocrat. 
F Yeah Lolita and 
Rosa Nitada both have excellent overviews to aristocrat. While ouji and 
dandy share eras of influence, the former adds fantasy and  child-like fun to a vest and knickerbockers, while the latter aims to be more  historically accurate with a full suit and top hat.
 Sometimes ouji outfits are used in 
visual kei, and visual kei elements can be used in ouji.
 Despite being very similar to each other, ouji is not
 considered
 lolita.  It is incorrect to call it "ouji lolita" or "kodona lolita" because  lolita requires a specific silhouette made by wearing a petticoat. "Boy  lolita" or "brolita" are not correct either because they refer to 
a male wearing lolita.
 
Why not kodona?
Although the term is widely used, "kodona" is not the proper term for the style. The term kodona came from an interview with 
Plastic Tree's vocalist Ryutaro. The term was used to describe his personal style. A direct translation:
kodona = kodomo + otona → childult = child + adult 
As you can see, childult isn't the best term to describe the style and it would be odd to go around saying "I'm wearing childult fashion." The international lolita community somehow adopted the term kodona and it just stuck. If you were to go to Japan and ask about kodona fashion, most likely 
they would have no idea what you are talking about. 
  | 
| Ryutaro's interview from Gothic & Lolita Bible Vol. 1 | 
Stay tuned for part 2 Basic Anatomy of an outfit!