A secosteroid is a molecule similar to a steroid

Steroid

A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

 but with a "broken" ring. Etym: L. Seco, to cut, _ steroid

Secosteroids are very similar in structure to steroids except that two of the B-ring carbon atoms (C9 and 10) of the typical four steroid rings are not joined, whereas in steroids they are.

In humans, the most important secosteroid is Vitamin D

Vitamin D

Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

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