Other forms: solidi
When someone uses the word solidus, they're either talking about an ancient gold coin and/or the punctuation mark also known as a slash.
Solidus comes from the Late Latin nummus solidus, "solid coin," a reference to thick Byzantine Empire coins made of pure gold. The word shilling evolved from solidus, and later the slash between shillings and pence in English prices (eventually replaced by a decimal point) also came to be called a solidus. You use a solidus whenever you write a date this way: 6/19/1805, or a choice between two options like this: vanilla/chocolate.