It's much more than just a plant
The term “hemp” is used to mean cannabis that contains 0.3 percent or less THC content by dry weight.
Why 0.3 percent? This definition was first proposed in 1979, in a book called “The Species Problem in Cannabis: Science & Semantics.” In the book, author Ernest Small addresses the fact that it’s difficult to distinguish hemp and cannabis because there’s no actual taxonomical difference between the two.
Small proposed the 0.3 percent rule as a possible solution, but he himself acknowledged that it’s an arbitrary number.
This number was used in the legal definition of hemp, as specified in the Agricultural Act of 2018 and other laws in the United States. Because the THC level in hemp is so low, it’s unlikely to get you high.