hairlineEnglishOxford New American Dictionaryhairline /ˈherˌlīn, ˈhɛrˌlaɪn / ▸ noun 1 the edge of a person's hair, especially on the forehead. 2 a very thin or fine line: [as modifier] a hairline fracture the boards fitted so tightly together, there was only a hairline between them.