Definition

In botany, the term herb refers to a herbaceous plant, defined as a small, seed-bearing plant without a woody stem in which all aerial parts (i.e. above ground) die back to the ground at the end of each growing season. Usually the term refers to perennials, although herbaceous plants can also be annuals (where the plant dies at the end of the growing season and grows back from seed next year), or biennials. This term is in contrast to shrubs and trees which possess a woody stem. Shrubs and trees are also defined in terms of size, where shrubs are less than ten meters tall, and trees may grow over ten meters. The word herbaceous is derived from Latin herbāceus meaning "grassy", from herba "grass, herb".

Another sense of the term herb can refer to a much larger range of plants, with culinary, therapeutic or other uses. For example, some of the most commonly described herbs such as sage, rosemary and lavender would be excluded from the botanical definition of a herb as they do not die down each year, and they possess woody stems. In the wider sense, herbs may be herbaceous perennials but also trees, subshrubs, shrubs, annuals, lianas, ferns, mosses, algae, lichens, and fungi. Herbalism can utilize not just stems and leaves but also fruit, roots, bark and gums. Therefore, one suggested definition of a herb is a plant which is of use to humans, although this definition is problematic since it could cover a great many plants that are not commonly described as herbs.

Ancient Greek philosopher Theophrastus divided the plant world into trees, shrubs, and herbs. Herbs came to be considered in three groups, namely pot herbs (e.g. onions), sweet herbs (e.g. thyme), and salad herbs (e.g. wild celery). During the seventeenth century as selective breeding changed the plants size and flavor away from the wild plant, pot herbs began to be referred to as vegetables as they were no longer considered only suitable for the pot.