Secret World
A secret world (SW) is any location in a Metroid Prime game where it's possible to get out of bounds. Some of the more well-known secret worlds which are useful in Metroid Prime speedruns are the one in Gathering Hall, which is used in the Ice Beam Before Flaahgra (IBBF) wallcrawl, and the one in Phendrana Shorelines, which is used in the Wave/Sun wallcrawl.
For a complete list of secret worlds, click the respective links below:
Name Origin
The term "secret world" was originally coined by players of Metroid II when they found ways to get out of bounds, leaving rooms jumbled and leading them to believe they had discovered "secret worlds."
Legality
Secret worlds are currently allowed in every category unless explicitly banned. This was not always the case, however, as secret worlds were banned on SpeedDemosArchive for years before eventually being legalized. I personally like watch much more speedruns without used secret worlds because these speedruns are real.