substance produced by a plant that. in low concentrations. affects physiological processes such as growth and development. often at a distance from the substance’s point of origin (see plant growth regulator).

Merriam-Webster Online Dictionary
plant hormone (noun)
an organic substance other than a nutrient that in minute amounts modifies a plant physiological process , especially one produced by a plant and active elsewhere than at the site of production
plant hormone (Wikipedia)

Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pathogen defense, stress tolerance and through to reproductive development. Unlike in animals (in which hormone production is restricted to specialized glands) each plant cell is capable of producing hormones. Went and Thimann coined the term "phytohormone" and used it in the title of their 1937 book.

Lack of the plant hormone auxin can cause abnormal growth (right)

Phytohormones occur across the plant kingdom, and even in algae, where they have similar functions to those seen in vascular plants ("higher plants"). Some phytohormones also occur in microorganisms, such as unicellular fungi and bacteria, however in these cases they do not play a hormonal role and can better be regarded as secondary metabolites.

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