It can refer to several things:
* A seal, stamp, or mark used for authentication. This is the most common meaning. It can refer to a physical seal used to authenticate documents or to a digital signature.
* A symbol or emblem. This is a more general meaning and can refer to any kind of distinctive mark or symbol.
* A magical sigil. This is a specific kind of symbol used in magic and other occult practices.
The specific meaning of "sigillum" will depend on the context in which it is used.