1. A traditional Japanese school uniform worn by girls, primarily in middle school and high school, but sometimes elementary school, that borrows scaled down design elements from the sailor suits worn by children of European royal familie during the 19th and 20th centuries.
2. Japanese. Seifuku or "競服" literally translates to "school clothes" and applies to girl's and boy's uniforms (thanks Nate).
EX. Arguably, the most famous wearers of sailor fuku (at least in America) are Sailor Moon and the Sailor scouts of the popular 90's sentai anime/manga that found popularity in the West. Many anime have cliched uniforms but others can be identified by the distinct uniform worn by the teenage characters that can be quite distinct; for example, the black and white uniforms from Bleach versus the colorful uniforms worn by the characters of Yu Yu Hakusho versus the more intricate or generic designs below.
Surely, there is nothing better to wear when trying to fight crime, slay demons, or save the world...