业余 (yè yú) is an adjective meaning 'amateur, non-professional' or 'spare-time, after-hours.' It describes someone who does something as a hobby rather than as a job, or time outside of work. Unlike 外行 (wài háng), which means 'layperson' and implies a lack of expertise, 业余 focuses on the activity being done in one's free time, not necessarily on skill level.