Hairline is the edge where your hair meets your forehead. If you look in a mirror and trace an imaginary line across the top of your forehead where your hair starts growing, that's your hairline. Everyone's hairline has a slightly different shape: some form a gentle curve, others make more of a V shape in the middle (called a widow's peak), and some run almost straight across.

The word also describes something extremely thin or fine. A hairline crack in a wall or dish is so narrow you can barely see it, no thicker than a single hair. A hairline fracture in a bone is a tiny crack that might not even show up clearly on an X-ray. When engineers inspect bridges or airplane parts, they look carefully for hairline cracks that could become dangerous if they grow larger.

As people age, their hairline sometimes moves backward on their forehead, something called a receding hairline. This is a normal part of getting older for many adults, though some people keep the same hairline their whole lives.