Abbreviation of September. (month)
"The satellite detected a surge in SEP activity that could disrupt power grids across the northern hemisphere."
Initialism of sensory evoked potential.
Initialism of solar energetic particles.
In plain English: Sep is an old-fashioned way of saying September, which is the ninth month of the year.
"The sep was busy sorting mail at the local post office."
September
"The flight was delayed, so we won't arrive until late sep."
Sep is an informal abbreviation derived from English, originally shortening the word September to refer specifically to that month. It entered common usage in modern times as a convenient shorthand rather than having a distinct historical root separate from its parent term.