sideline

/ˈsaɪdlaɪn/ noun [C]

Definition

› a business activity that someone does in addition to their main business: For many companies, producing own-label goods is not a sideline, it's a strategy for growth.a sideline in sth The company has been developing a sideline in closed-circuit television since buying the electronics business last year.

on the sidelines › not being used or not involved in an activity: The absence of new products is keeping consumers on the sidelines. stay/wait/sit on the sidelines

(Definition of sideline noun from the Cambridge Business English Dictionary © Cambridge University Press)