the act of working at an extra job, especially without telling your main employer

You'll lose your job if the boss finds out you've been moonlighting. 如果老板发现你在兼职捞外快，你的工作就保不住了。

Sometimes, when you want to change direction or move into a new industry, moonlighting is the only option. 有时为了转行或是尝试新的行当，只能从兼职干起。