With the term basionym, word derived from the Greek basis = base and onoma = name, which in the literal sense identifies the name that was the basis from which another name was formed.
The use of the nomenclature is regulated by the International Nomenclature Code for Algae, Fungi and Plants (ICN), which came into effect on January 1, 2012, and has made some substantial changes compared to the previous International Botanical Nomenclature Codices.
Furthermore, the International Nomenclature Code for Algae, Fungi, and Plants contains the new rules for the establishment of valid taxa.
In the botanical nomenclature, basionymology represents the most ancient, validly and legitimately published binomial from which the specific epithet was used to serve as the basis for a new combination, following a transfer to another genus or to another taxonomic rank.
For this reason the basionym must necessarily consist of a pre-existing binomial and since this binomial must be legitimized according to the rules of the ICN, it does not necessarily correspond to the oldest binomial published in literature.
For example, spruce is known today as Picea abies (L.) H. Karst. It was transferred to the Picea genus by the botanist Karsten, who based the new binomial of his invention on the previous name Pinus abies L., which had been published by Linnaeus.
Therefore, Pinus abies L. is the basionimo of Picea abies (L.) H. Karst.