Thorn/θɔːrn/ Definition 1 of 3Pro Tip 1/3Part of Plant Thorn specifically refers to sharp extensions on plants, not sharp objects in general. Roses and cacti are common plants known for their thorns. Pro Tip 2/3Painful Touch Thorns can prick your skin, so approach with caution when dealing with thorny plants. I accidentally pricked my finger on a thorn while gardening. Pro Tip 3/3Natural Defense Plants have thorns as a defense mechanism to protect from herbivores. The thorns on that bush protect it from being eaten by animals.