noun
sideline
SYDE-lyne/ˈsaɪdlaɪn/
An activity done in addition to one's main work.
She sells handmade pottery as a sideline to her office work.

Etymology
A line beside the main one became a business carried on beside one's regular work.
More examples
- Teaching pottery provides a useful sideline.
- Her photography begins as a sideline.
Forms
sideline
Similar words
- side job
- Additional paid work beyond the main occupation.
- secondary pursuit
- An activity subordinate to a principal one, not necessarily paid.