the.com/story arc

means The shaped progression of events and change a character or plot travels through, from setup to payoff, across a story or series.

from Arc here is a straight borrow from geometry and physics — a curved trajectory, like the path of a thrown ball. Storytellers started using it to describe a plot's rise and fall roughly in the 20th century, especially once television writers needed a word for a thread that stretched across multiple episodes rather than resolving in one. The physical image stuck because it's apt: a story, like a thrown object, goes up, bends, and comes down somewhere changed.