Definition of the front line noun from the Oxford Advanced American Dictionary

 

the front line

 noun
noun
 
[singular]
 
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  • an area where the enemies are facing each other during a war and where fighting takes place Tanks have been deployed all along the front line. front-line troops
  • Idioms
    on the front line (of something)
     
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    doing work that will have an important effect on something a life spent on the front line of research
    See the Oxford Advanced Learner's Dictionary entry: the front line