It can refer to:
* A physical seal: This is the most common meaning, and it refers to a stamp or impression made on a document or object to authenticate it.
* A metaphorical seal: This can refer to something that secures or protects something else, like a promise or a treaty.
The word "sicillum" is a diminutive of "sigillum", which also means "seal".
Note: The word "sicillum" is rarely used in modern English. It's more common to use the word "seal" or "signet" instead.