Overview
Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials.
Definitions of "herb" and "herbaceous"
The fourth edition of the Shorter Oxford English Dictionary defines "herb" as:
"A plant whose stem does not become woody and persistent (as in a tree or shrub) but remains soft and succulent, and dies (completely or down to the root) after flowering";
"A (freq. aromatic) plant used for flavouring or scent, in medicine, etc.". (See: Herb)
The same dictionary defines "herbaceous" as:
"Of the nature of a herb; esp. not forming a woody stem but dying down to the root each year";
"BOTANY Resembling a leaf in colour or texture. Opp. scarious".
Botanical sources differ from each other on the definition of "herb". For instance, the Hunt Institute for Botanical Documentation includes the condition "when persisting over more than one growing season, the parts of the shoot dying back seasonally". Some orchids, such as species of Phalaenopsis, are described in some sources (including the authoritative Plants of the World Online) as "herbs" but with "leaves persistent or sometimes deciduous". In the glossary of Flora of the Sydney Region, Roger Charles Carolin defines "herb" as a "plant that does not produce a woody stem", and the adjective "herbaceous" as meaning "herb-like, referring to parts of the plant that are green and soft in texture".
From Wikipedia (CC BY-SA 4.0).