I. side ‧ line 1 /ˈsaɪdlaɪn/ BrE AmE noun
1 . [countable] an activity that you do as well as your main job or business, in order to earn more money
as a sideline
Zoë does a bit of freelance photography as a sideline.
2 . on the sidelines not taking part in an activity even though you want to or should do
stand/stay/remain etc on the sidelines
You can’t stay on the sidelines for ever; it’s time you got involved.
3 . sidelines [plural] the area just outside the lines that form the edge of a sports field
on the sidelines
Wenger stood on the sidelines shouting instructions.
4 . [countable] a line at the side of a sports field, which shows where the players are allowed to play
II. sideline 2 BrE AmE verb [T, usually passive]
if you are sidelined, you are unable to play in a sports game because you are injured, or unable to take part in an activity because you are not as good as someone else:
Owen was once again sidelined through injury.