hairline Syllabification: (hair·line)Pronunciation: /ˈhe(ə)rˌlīn/Translate hairline | into German | into Italian | into Spanish Definition of hairline noun 1the edge of a person’s hair, especially on the forehead. 2a very thin or fine line:the boards fitted so tightly together, there was only a hairline between them [as modifier]:a hairline fracturehairline in other Oxford dictionaries Definition of hairline in the British & World English dictionary