Sedition is the illegal act of inciting people to resist or rebel against the government in power. It's what the southern states did at the start of the Civil War.
Sedition is the rebellious talk and encouragement that might lead to a mutiny, and can be charged as a crime, like treason. The Alien and Sedition Acts passed into law in the late 1700s were challenged by none other than Thomas Jefferson as a violation of Americans' First Amendment rights to free speech. The Sedition Act quietly expired a few years later but was essentially replaced in the early 1900s by what is called the Espionage Act, which was later repealed as well. Proving sedition can become a murky business. When should the expression of one's activism be considered "free speech" and when should it be considered sedition?