1. n. Abbreviation for 'semicolon', when speaking.
"Commands to grind are prefixed by semi-semi-star" means that the prefix is ';;*', not 1/4 of a star.
2. A prefix used with words such as 'immediately' as a qualifier.
"When is the system coming up?"
"Semi-immediately." (That is, maybe not for an hour.)
"We did consider that possibility semi-seriously."
See also infinite.