I. ˈ ̷ ̷| ̷ ̷ adjective
1. : available for immediate effective combat service
first-line troops
first-line ships
2. : of the first quality or importance
there are in the area no potential first-line industrial centers — S.G.Hanson
a first-line tire
attended by 50 first-line business executives — American Standards Association
II. ˈ ̷ ̷ ˈ ̷ ̷ adjective
: being the preferred, standard, or first choice
first-line treatment of tuberculosis