A more thorough explanation:

actus legitimus (ak-tuh-s luh-jit-uh-muhs). [Law Latin] In legal history, an act that is recognized by the law; a legal act; specifically, an act that was performed with solemn rituals.

• The signing of a marriage certificate is an example of an actus legitimus.
• The swearing-in of a president is another example of an actus legitimus.

These examples illustrate the definition of actus legitimus because they are both legal acts that are recognized by the law and are performed with solemn rituals. The signing of a marriage certificate is a legal act that formalizes a marriage, and it is typically accompanied by a ceremony. The swearing-in of a president is a legal act that marks the beginning of a presidential term, and it is accompanied by a public ceremony.