Ombre hair refers to hair color fading from dark to light, or vice versa. Instead of the color starting from the roots, it gradually becomes lighter, or darker, towards the ends. The word "ombre" in French means shadow or shade. Think Sarah Jessica Parker in Sex and the City, Rachel Bilson, Whitney Port.
Friend 1: "Did you notice all the celebrities are letting their roots grow out?"
Friend 2: "Yes, that's called ombre hair!"