Nihonshudo (日本酒度) is often glossed "
sake meter value"
in English, a remarkably
content-free name. Unfortunately, it
really is a pretty exact translation, as "
nihonshu" (lit.
Japanese
alcohol) is how the Japanese refer to sake and "do" means
"
degree" (and yes, this is used for 25-degree temperatures,
90-degree angles and so on as well).
What it really is, however, is a measurement of the relative
density of the sake, also known as specific gravity if you want
to sound even more scientific. This would seem to be a pretty strange
thing to worry about, but it turns out that density is directly
correlated to the amount of residual unfermented sugar in the
sake. As nihonshudo is a relative scale based on water at
0, positive numbers -- low density --
mean dry (karakuchi) sake and negative numbers --
high density -- mean sweet (amakuchi) sake.
Do note that these are only general guidelines, many other things like
acidity affect the perceived dryness of the drink.
"Normal" sake (ie. sake produced with a normal fermentation process)
will have a nihonshudo around +2. Ratings above +6 or so
are considered dry and ratings above +10 are considered rather silly,
although I've once sampled a fairly ludicrous +25. Negative values,
on the other hand, are not often seen, with the notable
exception of unfiltered nigorizake.
Nihonshudo is included on a standard Japanese sake label as
the fifth row from the top.