A quitter is someone who stops in the middle of a hard thing and does not finish it — the word says they did not have the determination, or the courage, to see it through. You hear it in sport, school, and pep talks: don’t be a quitter. The verb quit can be quite neutral (“she quit the job”), but the noun quitter is usually unkind. It judges character, not just a choice. Parents, coaches, and angry friends use it; careful, kind speech often avoids it, or puts it in quotes, because it can crush a student who is tired rather than weak. Though actually, people also use it about themselves, half as a joke, half as a push: I’m no quitter. It is informal. In a formal report you would more likely say that someone withdrew or did not complete the task.