A dimple was a cleft in one's flesh.
Dimples were often considered cute. In 2374, the holographic character Ginger described Liberace as having "the sweetest dimples." (DS9: "His Way")
According to Harry Kim, the way to tell otherwise identical twin sisters Megan and Jenny Delaney apart was that unlike Jenny, Megan had a dimple on her right cheek. (VOY: "Thirty Days")