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a combining form meaning “tenth,” used in words denoting units of the metric system (deciliter); on this model, extended to other systems (decibel).
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Compare deca-.

Origin of deci-

<French déci-<Latin decimus tenth
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2021


What does deci- mean?

Deci- is a combining form used like a prefix meaning “tenth.” It is most often used to denote units of the metric system.

Deci- comes from the Latin decimus, meaning “tenth.” The word decimate, meaning “to destroy a great number or proportion of” comes from this same Latin root. Find out how at our entry for decimate.

A similar word from Greek, déka, meaning “ten,” gives us the combining forms dec-, deca-, and deci-. Want to know more? Check out our Words That Use articles for all three forms.

Despite the similarity between the two forms, it’s important not to confuse deci-, which means “tenth,” with deca-, meaning “ten.” A decigram is one-tenth of a gram (0.1 g), but a decagram is ten grams (10 g).

Examples of deci-

One example of a technical term you may have encountered that features the combining form deci- is decimeter, “a unit of length equal to 1/10 of a meter.” Decimeter comes from the French décimètre, which uses the equivalent of the combining form deci-.

The first part of the word, deci-, means “tenth,” as we have seen. The combining form -meter has a variety of meanings, but in this case it refers to the metric unit “meter.” Decimeter literally means “a tenth of a meter.”

What are some words that use the combining form deci-?

What are some other forms that deci- may be commonly confused with?

Not every word that begins with the exact letters dec- or deci-, such as decide and decision, is necessarily using the combining forming deci- to denote “tenth.” Learn where these words come from at our entries for decide and decision.

Break it down!

A liter is a unit used for measuring liquids. With this in mind, what is a deciliter?

How to use deci- in a sentence

British Dictionary definitions for deci-


denoting one tenth; 10 –1decimetre Symbol: d

Word Origin for deci-

from French déci-, from Latin decimus tenth
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for deci-


One tenth (10-1):decibel.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for deci-


A prefix that means “one tenth,” as in deciliter, one tenth of a liter.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.