noun
principal occupation; core business; main job
Refers to one's primary profession or the main line of business, as opposed to a side job or secondary activity.
彼の本業は教師ですが、週末はカフェでアルバイトをしています。
His main job is a teacher, but he works part-time at a café on weekends.
当社の本業はソフトウェア開発です。
Our company's core business is software development.