A(n) herb (pronounced /ˈɝb/ or /ˈhɝb/; see pronunciation differences) is a plant that is valued for qualities such as medicinal properties, flavor, scent, or the like.

Herbs have a variety of uses including culinary, medicinal, or in some cases even spiritual usage. The green, leafy part of the plant is often used, but herbal medicine makes use of the roots, flowers, seeds, root bark, inner bark (cambium), berries and sometimes the pericarp or other portions. General usage differs between culinary herbs and medicinal herbs. A medicinal herb may be a shrub or other woody plant, whereas a culinary herb is a non-woody plant, typically using the leaves. Any of the parts of the plant, as well as any edible fruits or vegetables, might be considered "herbs" in medicinal or spiritual use.

Culinary use of the term "herb" is much more specific and narrow. Culinary use typically distinguishes between herbs, the leafy green parts of the plant, and spices, all the other parts of the plant, including seeds, berries, bark, root, fruit, and even occasionally leaves. Culinary herbs are distinguished from vegetables in that they are used in small amounts and provide flavor (similar to spices) rather than substance to food.

Any plant contains numerous phytochemicals that have varying effects on the body. Even when consumed in the small levels that typify culinary "spicing", there may be some effects, and some herbs are toxic in larger quantities. For instance, some types of herbal extract, such as the extract of Hypericum perforatum (St. John's wort), or the Piper methysticum (kava plant) can be used for medical purposes to relieve depression and stress. But high amounts of these herbs may lead to poisoning, and should be used with caution.