The term demi-decade began to appear more widely in early 2015 in the context of retrospective commentary (e.g. British film critic Peter Bradshaw’s pick of the top 50 films of the demi-decade, see the citation above), though it pops up in other domains too (e.g. a ‘demi-decade’ day for alumni of a University of Oxford college). Though it has particular relevance in 2015, the word has been used, albeit pretty marginally, for a number of years as a reference to five-year intervals, and a trawl of the internet gives evidence for its occurrence as far back as 1824.