A medicinal plant was any plant which had medicinal properties.
In 2375, The Doctor reported that many of the medicinal plants Neelix had collected were destroyed. Neelix assured him that he still had a reserve supply of herbs he could use. When the Doctor asked if he had been holding out, the Talaxian replied that he saved them in case of just such an emergency. The Doctor commended him for planning ahead. (VOY: "Latent Image")