Definition of vitamin D
: any or all of several fat-soluble vitamins chemically related to steroids, essential for normal bone and tooth structure, and found especially in fish-liver oils, egg yolk, and milk or produced by activation (as by ultraviolet irradiation) of sterols: such as
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Merriam-Webster unabridged
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Time Traveler for vitamin D
The first known use of vitamin D was circa 1921
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