A hormone (from Greek ορμή - "to set in motion") is a chemical messenger from one cell (or group of cells) to another. All multicellular organisms produce hormones (including plants - see article phytohormone).
The concept of internal secretion developed in the 19th century; Claude Bernard described it in 1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognised and even treated adequately (e.g., hypothyroidism with extract of thyroid glands).
The major breakthrough was the identification of secretin, the hormone secreted by the duodenum that stimulates pancreatic secretions, by Ernest Starling and William Bayliss in 1902. Previously, the process had been considered (e.g., by Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss demonstrated that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger.
Starling is also credited with introducing the term hormone, having coined it in a 1905 lecture. Later reports indicate it was suggested to him by the Cambridge physiologist William B. Hardy (Henderson 2005).
The remainder of the 20th century saw all the major hormones discovered, as well as the cloning of the relevant genes and the identification of the many interlocking feedback mechanisms that characterise the endocrine system.