What is defined as the act of betraying one's country?

The act of betraying one's country is defined as treason. This term specifically refers to actions that undermine the security or sovereignty of a nation, such as waging war against it or providing aid and comfort to its enemies. Treason is considered one of the gravest offenses in many legal systems, often carrying severe penalties due to the threat it poses to the state and its citizens.

Libel and slander pertain to defamation, involving false statements that damage a person's reputation, rather than the betrayal of a nation. Indictment refers to a formal accusation of a crime, often related to legal proceedings, and does not imply betrayal or treachery against the state. Thus, the correct choice is directly tied to the specific definition of treason as it relates to national loyalty and security.