A type of film that is applied to the inside of a cars windows to make them darker. Tint has many benefits including more privacy, UV protection (so your car interior doesn't fade as fast), and it'll keep your car cooler on the hot summer days. Tint is measured in percentage, the lower the percentage the darker the tint.
Also very commonly referred to incorrectly as "tints" by idiots who don't know the difference between singular and plural. Usually the same people that call geese "gooses".