The trajectory of change for a character, subplot, or story thread from its starting point to its conclusion.
A story arc is the path of transformation that a character, relationship, or storyline follows over the course of a narrative. While narrative arc refers to the overall shape of the entire story, story arc is often used more specifically to describe the journey of a single element within it. Your protagonist has an arc. Your villain has an arc. Your romance subplot has an arc. Each one traces a line from a starting condition through conflict and change to some kind of new equilibrium. A great story weaves multiple arcs together so they reinforce and complicate each other.
Story arcs are what make readers care. Plot events are just things that happen, but arcs give those events meaning by connecting them to change. When a character grows, falls, learns, or refuses to learn, the reader feels the weight of everything that led to that transformation. If your story feels eventful but emotionally hollow, it is probably because the arcs are underdeveloped. Events without arcs are just a sequence of stuff. Arcs turn stuff into story.
Scrooge's arc is one of the most famous in literature: from cold-hearted miser to generous, joyful human being. The three ghosts are the mechanism, but the arc is the point.
Gatsby's arc is tragic because he cannot change. His inability to let go of the past defines his trajectory from hopeful dreamer to doomed romantic.
Woody's arc from jealous, possessive toy to generous friend is a masterclass in positive change. Every event in the plot serves this transformation.
Briony's story arc spans decades as she moves from a child whose imagination causes devastating harm to an aging writer desperately trying to make amends through fiction.
Plot is the sequence of events. Arc is the trajectory of change those events produce. Ask yourself: because of everything that happened, how is this character different at the end? That difference is the arc.
Not every character needs to grow in a positive direction. Negative arcs, flat arcs, and ambiguous arcs are just as powerful when they serve the story's themes.
A character's transformation should be driven by specific story events that challenge their beliefs. If they just suddenly 'get it' without sufficient pressure, the arc feels unearned.
Supporting characters need arcs too, even if they are smaller. A story where only the protagonist changes feels lopsided. Give your key secondary characters their own trajectories.
Choose a character from your work in progress and write two short paragraphs: one describing who they are on page one, and another describing who they are on the last page. Then list the three to five key events that drive the transformation between those two states. If you cannot identify those events, your arc might need more structure.