The term 'long thin mark' can be summarized as a 'STREAK, ' which is a six-letter word. A STREAK is defined as a long, thin line or mark, usually made by a continuous series of repeated movements or events. Streaks are often seen in nature, such as the streaks of lightning in a thunderstorm, or the streaks of color in a rainbow. Streaks can also be found in art, such as the streaks of paint on a canvas.