Selenium (noun)

1. A chemical element with the atomic number 34, a grayish-black, lustrous, metallic semimetal, sometimes used in alloys and as a photoelectric material.
2. A mineral form of selenium, used in the preparation of pigments and in the glass industry.
3. An essential trace element required for good health, especially for the thyroid gland, but potentially toxic in large amounts.

Origin:

Mid 19th century: from selene (greek for "moon") + -ium.

Examples:

1. Selenium is a trace element that is necessary for good health, but toxic in large amounts.
2. Selenium is used in the manufacturing of photovoltaic cells and other electronic devices.
3. Selenium is an essential component of many enzymes, including those involved in antioxidant defense and thyroid hormone metabolism.
4. Selenium deficiencies can lead to a range of health problems, including cardiovascular disease and impaired immune function.
5. Selenium is also used in the glass industry to decolorize glass and to improve its electrical conductivity.