Secession describes is the political movement to withdraw a bloc of membership from an organization or especially a section of territory and resident population from an existing state or sub-national unit of government. The Scottish nationalist secession movement is organized as the Scottish National Party.
Historically, the term "secessionist" is used more often to describe the losing Confederate side in the American Civil War or the War to Preserve Slavery. The present day supporters of the losers of that conflict are also known as the proponents of the Lost Cause of the South. During the Civil War era in America the secessionists were known as "seceshers" and are called such today. Modern era seceshers in the U. S. may include both Southerners (or as they're known to call themselves, "Southroners") and Northerners. All are afflicted by the "false-consensus effect" whereby they overestimate the importance and the popularity of their endeavor.
Even in America, it's not quite dead.
A common historical misrepresentation made by today's small community of active secession advocates is that the U.S. was born by an act of secession. This lie pops up on virtually all of their websites. While a group may attempt to secede from a voluntary association, an act of separation made by a colony is a rebellion and those framing the secesher narrative know this. So why believe anything else they have to say?