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.
half year noun
1: one half of a year (as January to June or July to December)
— half–yearly adverb or adjective
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.
Reference: Oxford Advanced Learner's Dictionary