四半期 (しはんき) is a common word in news reports, and it becomes much more common every January when the media analyzes various data from the previous year.
But there is something strange about this word. 半期 means "half year", and I've previously discussed how it's used in the terms 上半期 and 下半期. So when we see it used with a prefixed 四, we experience a slight mind-bend. Is it "four halves"? That doesn't make any sense.
In fact, 四半期 means one quarter of a year (or three months). When you see it, it means that the article you're reading is discussing quarterly data.
The idea that four (四) times one half (半期) somehow equals one quarter feels outright bizarre. And it is. Because the compound 四半期 isn't made of "四 and 半期"; it's actually "四半 and 期".
As a standalone word, 四半 means "quarter". Of course, this still appears to leave us with the literal meaning of "four halves", but the character 半 also originally means "equal division in two". So the meaning of "quarter" may have come from the idea of dividing something in half and then repeating that division, which gives us four quarters. This 四半 was then suffixed with 期, and the combination gives us the meaning of "quarter period".
All of this Japanese pseudo-math can really make one's head spin, but there's no need to overthink this naughty little word. Just make a mental note that the 四 is linked to four equal parts, and you're all set for reading the newspapers' next quarterly reports.
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