The definition of lithium is an alkali metal with the lowest density of any solid element.
An example of lithium is what is added to molten glass to make it stronger and lighter.
See lithium in Webster's New World College Dictionary
Origin: ModL: name coined (1818) by Baron Jöns Jakob Berzelius for element discovered (1817) by his student A. Arfwedson (1792-1841), Swed chemist < lithia + -ium
See lithium in American Heritage Dictionary 4
noun Symbol Li
Origin: From lithia.
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