Tannin/ˈtænɪn/ Definition 1 of 3Pro Tip 1/3Tanning Substance Tannin is key in turning animal hides into durable leather. Quality leather often comes from excellent tannin treatment. Pro Tip 2/3Bitter Taste Tannin can make plants taste bitter, deterring herbivores. The tannin in these leaves adds a bitter flavor. Pro Tip 3/3Natural Dye Tannin-rich plants are often used to create natural dyes. Tannin contributes to the dye's vibrant color.