A series of lawsuits challenging former President Donald Trump's ability to run for his old job raise the question of what, exactly, is an insurrection. A rarely used section of the Constitution says no one who swore an oath to the document but then “engaged in insurrection” against it can hold higher office.

Liberal groups have filed lawsuits in Colorado, Minnesota and other states to bar Trump from the ballot, citing a rarely used constitutional prohibition against holding office for those who swore an oath to uphold the Constitution but then “engaged in insurrection” against it. The two-sentence clause in the 14th Amendment has been used only a handful of times since the years after the Civil War.

Because of that, there’s almost no case law defining its terms, including what would constitute an “insurrection.” While people have argued about whether to call Jan. 6 an insurrection ever since the days following the attack, the debate in court this week has been different — whether those who ratified the amendment in 1868 would call it one.

“There’s this very public fight, in all these colloquial terms, about whether it’s an insurrection, but it really comes down to brass tacks defining what this constitutional term means,” said Derek Muller, a Notre Dame law professor who’s followed the litigation closely.