A medicinal plant is a plant used for its particular properties that are beneficial to human or even animal health.

In the Middle Ages in medieval medicine, it was defined by the French pharmacopoeia as a “plant drug within the meaning of the European pharmacopoeia, at least part of which has medicinal properties”. A “herbal drug” is (among other things) a plant or part of a plant, used as it is, either most often in dried form, or in a fresh state.

The expression plant drug or, more commonly, drug, therefore designates — historically — a natural raw material used in the manufacture of drugs. Nowadays, the word is equivocal and some have proposed that it be, in the context of medicinal plants, replaced by the expression "part of plant used".

The plant is rarely used whole (hawkweed). Most often it is a part of the plant: rhizome (ginger), bulb (squill), root (angelica), aerial parts (nettle), stem (horsetail), bark (cinnamon), bud (pine) , leaf, (sage), flowering top (loosestrife), flower (violet), petal (poppy), fruit (fennel), seed (flax), seed coat (ispaghul), plant exudation (gum arabic, myrrh) , thallus of algae (kelp). Different parts of the same plant can have different uses (sapwood and linden inflorescence).

Plants having medicinal properties can also have food or condiment uses or even be used for the preparation of hygienic drinks. Before modern times, the theory of signatures played a great role in distinguishing by analogy the plants necessary for human healing.

According to WHO data, 14 to 28% of plants worldwide are listed as having a medicinal use. Surveys carried out in the year 2000 reveal that 3 to 5% of patients in Western countries, 80% of rural populations in developing countries and 85% of populations south of the Sahara use medicinal plants as their main treatment.