A thorn was a spiny projection used as a defense mechanism by plants.
While most thorns grew on the body of plants, some plants contained thorns within their flowers, and shot them at passers-by. Plants with thorns of this kind included the pod plants of Gamma Trianguli VI and a plant once kept in Lab 4005 aboard the USS Cerritos. (TOS: "The Apple", LD: "The Spy Humongous")
When Captain James T. Kirk brought Miranda Jones to the arboretum of the USS Enterprise, she pricked her finger on the thorn of a rose. When Jones left the Enterprise, Kirk presented her with another rose. She asked, "I suppose it has thorns?", and Kirk replied, "I never knew a rose that didn't." (TOS: "Is There in Truth No Beauty?")
In 2269, Hikaru Sulu initially attributed his pain upon touching a Retlaw plant to a thorn, when in reality, it had bitten him. (TAS: "The Infinite Vulcan")
The thorns of the predatory vine on Surata IV contained microbes which infected an individual. (TNG: "Shades of Gray")