Deca- or deka- (symbol da) is a prefix in the metric system, also a numerical prefix, denoting a factor of ten. The term is derived from the Greek δέκα meaning "ten". Its symbol is the only SI prefix that uses more than one character.
The prefix was a part of the original metric system in 1795. It is not in very common usage, although the decapascal is occasionally used by audiologists. The decanewton is also encountered occasionally, probably because it is an SI approximation of the kilogram-force. Its use is more common in Central Europe: in German, Polish (deka, deko), Czech, Slovak and Hungarian, deca is common (and used a word on its own always means decagram). A runway number typically indicates its heading in decadegrees.
Before the symbol was standardized as da with the introduction of the International System of Units in 1960, various other symbols were more common, such as "dk", "D", and "Da".
- The blue whale is approximately 30 meters or 3 decameters in length.
- The metric system was introduced in 1795 with six prefixes. The other dates relate to recognition by a resolution of the CGPM.
- Decamer, a thing composed of ten sub-units
- Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811
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