A part-timer is a person who works part-time: only some hours of the day, or only some days of the week, not a full working week. The word names the person, not the job itself. A shop might have three full-timers and two part-timers. Students, parents of young children, and people with a second job are often part-timers, but anyone can be. It is everyday and slightly informal; in careful notices you may see ‘part-time worker’ or ‘part-time staff’ instead. It is not about a clock or a kitchen timer. Opposite idea: a full-timer. Pay, holidays, and benefits are often less than for full-time staff, though that depends on the country and the contract — the word itself only talks about hours, not about whether the work is casual or serious.