Is there a word that means "a span of six months"? That is, I want to connote a stretch of time that lasts half a year, not an event that happens every six months.
The SAS (originally Statistical Analysis System) coins semiyear
( http://www.okstate.edu/sas/v8/saspdf/ets/chap3.pdf ), but this is hardly common usage.
Surely fmark takes pains to indicate that the adjective biannual (or, less ambiguous, semiannual) is not what he wants.
- You should receive a reply within seven days.
- Two elections were held within the space of a year.
Reference: Oxford Advanced Learner's Dictionary
You can try "six-month event".
You could use the word "a semester", I don't know if it goes well with what you are writting but for me that's a period of 6 months