Sideline/ˈsaɪdlaɪn/ Definition 1 of 3Pro Tip 1/3Extra Income A sideline is often a job or hobby that earns additional money besides the main source of income. She sells handmade jewelry as a sideline. Pro Tip 2/3Less Time Consuming Sidelines typically require less commitment and time than one's main job. His sideline as a music tutor fits perfectly into weekends. Pro Tip 3/3Passion Projects Sidelines can be related to personal passions or hobbies not pursued in one's career. Her passion for baking is now a profitable sideline.