Priority is a principle in biological taxonomy by which a valid scientific name is established based on the oldest available name. It is a decisive rule in botanical and zoological nomenclature to recognise the first binomial name (also called binominal name in zoology) given to an organism as the correct and acceptable name. The purpose is to select one scientific name as a stable one out of two or more alternate names that often exist for a single species.

The International Code of Nomenclature for algae, fungi, and plants (ICN) defines it as: "A right to precedence established by the date of valid publication of a legitimate name or of an earlier homonym, or by the date of designation of a type." Basically, it is a scientific procedure to eliminate duplicate or multiple names for a species, for which Lucien Marcus Underwood called it "the principle of outlaw in nomenclature".

The principle of priority has not always been in place. When Carl Linnaeus laid the foundations of modern nomenclature, he offered no recognition of prior names. The botanists who followed him were just as willing to overturn Linnaeus's names. The first sign of recognition of priority came in 1813, when A. P. de Candolle laid out some principles of good nomenclatural practice. He favoured retaining prior names, but left wide scope for overturning poor prior names.

During the 19th century, the principle gradually came to be accepted by almost all botanists, but debate continued to rage over the conditions under which the principle might be ignored. Botanists on one side of the debate argued that priority should be universal and without exception. This would have meant a one-off major disruption as countless names in current usage were overturned in favour of archaic prior names. In 1891, Otto Kuntze, one of the most vocal proponents of this position, did just that, publishing over 30000 new combinations in his Revisio Generum Plantarum. He then followed with further such publications in 1893, 1898 and 1903. His efforts, however, were so disruptive that they appear to have benefited his opponents. By the 1900s, the need for a mechanism for the conservation of names was widely accepted, and details of such a mechanism were under discussion. The current system of "modified priority" was essentially put in place at the Cambridge Congress of 1930.

By the 19th century, the Linnaean binomial system was generally adopted by zoologists. In doing so, many zoologists tried to dig up the oldest possible scientific names, as a result of which proper and consistent names prevailing at the time, including those by the eminent zoologists like Louis Agassiz, Georges Cuvier, Charles Darwin, Thomas Huxley, Richard Owen, etc. came to be challenged. Scientific organisations tried to establish practical rules for changing names, but not a uniform system.

The first zoological code with priority rule was initially formulated in 1842 by a committee appointed by the British Association. The committee included Charles Darwin, John Stevens Henslow, Leonard Jenyns, William Ogilby, John O. Westwood, John Phillips, Ralph Richardson and Hugh Edwin Strickland. The first meeting was at Darwin's house in London. The committee's report, written by Strickland, was implemented as the Rules of Zoological Nomenclature, and popularly known as the Stricklandian Code. It was not endorsed by all zoologists, as it allowed naming, renaming, and reclassifying with relative ease, as Science reported: "The worst feature of this abuse is not so much the bestowal of unknown names of well-known creatures as the transfer of one to another."

In zoology, the principle of priority is defined by the International Code of Zoological Nomenclature (4th edition, 1999) in its article 23:

The valid name of a taxon is the oldest available name applied to it, unless that name has been invalidated or another name is given precedence by any provision of the Code or by any ruling of the Commission [the International Commission on Zoological Nomenclature]. For this reason priority applies to the validity of synonyms [Art. 23.3], to the relative precedence of homonyms [Arts. 53-60], the correctness or otherwise of spellings [Arts. 24, 32], and to the validity of nomenclatural acts (such as acts taken under the Principle of the First Reviser [Art. 24.2] and the fixation of name-bearing types [Arts. 68, 69, 74.1.3, 75.4]).