Combining form. Before a vowel dec-.
[Greek deka ten.]
Used with the sense 'having ten, tenfold';
specially in names of units of measurement, used to denote a factor of ten.
There are a number of deca- words that mean 'pertaining to ten', such as decadal and decadic, and some very familiar words too, such as decade (although, if you consider decade to be oh so very last decade, you can also use decennium, which means exactly the same thing but sounds so much more refined); a decathlon is a commonly mispronounced sporting event in which decathletes engage in ten different events, the Decalogue is the Ten Commandments, and December is so called because it was the tenth month in Roman times.
Finally, and just to confuse you, the related word deci- means 'one tenth', such as in decibel, decimal and decimate, and the words decadent, decalcomania and decaffeinated have absolutely nothing to do with the number ten at all.
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