Direct Derivatives:
* December: The 12th month of the year (originally the 10th month in the Roman calendar).
* Decimal: A number system based on ten.
* Decade: A period of ten years.
Other Derivatives:
* Deciduous: Trees that lose their leaves in the autumn (from the Latin "decidere" meaning "to fall down," which is related to "decem").
* Decagon: A polygon with ten sides.
* Decameter: A unit of length equal to ten meters.
* Deci: A prefix meaning one-tenth (e.g., decimeter, deciliter).
* Decillion: A number equal to 10^33 (a one followed by 33 zeroes).
Related Words:
* Duodecimal: A number system based on twelve (from Latin "duodecim" meaning twelve).
* Centenary: A period of one hundred years.
* Millennium: A period of one thousand years.
These are just a few examples, and there are many other words related to "decem" in various languages.