noun
side job; avocation; work on the side
Refers to work or an occupation pursued in addition to one's main job, often for personal interest or extra income. Somewhat formal or literary.
彼は本業の傍ら、余業として翻訳をしている。
He does translation as a side job alongside his main work.
余業に精を出すあまり、本業がおろそかになってはいけない。
You must not neglect your main job by putting too much effort into your side work.