The literal translation is "side job," but it can also mean, "second job."
To say that you have a "side job" means that you have another job aside from your primary job, just to make a little extra money on the side.
EXAMPLE: I work at an elementary school as my primary job, but I teach English online as my side job, so I can make a little extra money on the side.
副業の直訳は"side job"ですが、"second job"（第二の仕事）ということも意味します。
"side job"があるということは、少し[余分](https://eikaiwa.dmm.com/uknow/questions/68582/)にお金を得るために、本業とは別に[仕事](https://eikaiwa.dmm.com/uknow/questions/37171/)があるということを意味します。
例文
I work at an elementary school as my primary job, but I teach English online as my side job, so I can make a little extra money on the side.
「私は本業として、小学校で働いていますが、それとは別で少しでもお金を得られるように副業でオンラインの英語教師をやっています。」
My dad works full time as an engineer, but he's always had side projects, like designing apps or starting a small company. This term may suggest that the "project" isn't as serious as one's full-time job, so if you take your business very seriously, I would recommend using one of the other terms mentioned above.
The terms subsidiary and secondary can both be used to describe a second job. Subsidiary meaning a job that is less important than your day job but supplementary or additional to your day job.
Secondary means that you have an extra job which is less important or second to your day job.
Occupation is the term used to describe work
People can have other ways to earn an income in addition to their primary job.
Self-employed means that you are your own boss, it is your company or project.
The word alternate means ''another.''
Passive income: an income that comes without much effort or activity.
"I like to keep my options open, I have a nine-to-five job as well as my own business and passive income work."
本業に加えて副業を持つ人もいますね。
'self-employed' は「自営業者」を指します。
'alternate' は 'another'（別の／もう一つの）という意味です。
Passive income: ほとんど働かなくても入ってくるお金。
"I like to keep my options open, I have a nine-to-five job as well as my own business and passive income work."
（いろいろな選択肢を持っておきたいので、自分の会社と不労所得の仕事に加えて、普通の仕事もしています）
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