A humanoid's hairline was the area on their head where their hair began.
Jean-Luc Picard was bald. In 2370, when he believed Jason Vigo to be his son, he jokingly stated that Jason would never look at his hairline the same way again. (TNG: "Bloodlines")
In 2373, when The Doctor and B'Elanna Torres met a hologram of Lewis Zimmerman, who had created The Doctor in his image, The Doctor joked that he could see where he got his hairline from, as both he and Zimmerman were nearly bald. (VOY: "The Swarm")