Webster's English Dictionary

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frontier

fron.tier \.fr*n-'ti(*)r, 'fr*n-., fra:n-', 'fra:n-.\ n [ME fronter, fr. MF 
   frontiere, fr. front] 1a: a border between two countries  obs  1b: a 
   stronghold upon a frontier  2a: a region that forms the margin of settled 
   or developed territory  2b: the farthermost limits of knowledge or 
   achievement  2c: a new field that offers scope for activity  - frontier aj