Prawn is a common name for small aquatic crustaceans with an exoskeleton and ten legs (which is a member of the order decapoda), some of which can be eaten.
The term "prawn" is used particularly in the United Kingdom, Ireland, and Commonwealth nations, for large swimming crustaceans or shrimp, especially those with commercial significance in the fishing industry. Shrimp that are present in this category often belong to the suborder Dendrobranchiata. In North America, the term is used less frequently, typically for freshwater shrimp. The terms shrimp and prawn themselves lack scientific standing. Over the years, the way they are used has changed, and in contemporary usage the terms are almost interchangeable.
The terms shrimp and prawn are common names, not scientific names. They are vernacular or colloquial terms which lack the formal definition of scientific terms. They are not taxa, but are terms of convenience with little circumscriptional significance. There is no reason to avoid using the terms shrimp or prawn when convenient, but it is important not to confuse them with the names or relationships of actual taxa.
According to the crustacean taxonomist Tin-Yam Chan, "The terms shrimp and prawn have no definite reference to any known taxonomic groups. Although the term shrimp is sometimes applied to smaller species, while prawn is more often used for larger forms, there is no clear distinction between both terms and their usage is often confused or even reverse in different countries or regions." Writing in 1980, L. B. Holthuis noted that the terms prawn and shrimp were used inconsistently "even within a single region", generalising that larger species fished commercially were generally called shrimp in the United States, and prawns in other English-speaking countries, although not without exceptions.
A lot of confusion surrounds the scope of the term shrimp. Part of the confusion originates with the association of smallness. That creates problems with shrimp-like species that are not small. The expression "jumbo shrimp" can be viewed as an oxymoron, a problem that does not exist with the commercial designation "jumbo prawns".
The term shrimp originated around the 14th century with the Middle English shrimpe, akin to the Middle Low German schrempen, and meaning to contract or wrinkle; and the Old Norse skorpna, meaning to shrivel up, or skreppa, meaning a thin person. It is not clear where the term prawn originated, but early forms of the word surfaced in England in the early 15th century as prayne, praine and prane. According to the linguist Anatoly Liberman it is unclear how shrimp, in English, came to be associated with small. "No Germanic language associates the shrimp with its size... The same holds for Romance... it remains unclear in what circumstances the name was applied to the crustacean."
Taxonomic studies in Europe on shrimp and prawns were shaped by the common shrimp and the common prawn, both found in huge numbers along the European coastlines. The common shrimp, Crangon crangon, was categorised in 1758 by Carl Linnaeus, and the common prawn, Palaemon serratus, was categorised in 1777 by Thomas Pennant. The common shrimp is a small burrowing species aligned with the notion of a shrimp as being something small, whereas the common prawn is much larger. The terms true shrimp or true prawn are sometimes used to mean what a particular person thinks is a shrimp or prawn. This varies with the person using the terms. But such terms are not normally used in the scientific literature, because the terms shrimp and prawn themselves lack scientific standing. Over the years the way shrimp and prawn are used has changed, and nowadays the terms are almost interchangeable. Although from time to time some biologists declare that certain common names should be confined to specific taxa, the popular use of these names seems to continue unchanged.
The terms shrimp and prawn originated in Britain. In the use of common names for species, shrimp is applied to smaller species, particularly species that are dorsoventrally depressed (wider than deep) with a shorter rostrum. It is the only term used for species in the family Crangonidae, such as the common shrimp or brown shrimp, Crangon crangon. Prawn is never applied to very small species. It is applied to most of the larger forms, particularly species that are laterally compressed (deeper than wide) and have a long rostrum. However, the terms are not used consistently. For example, some authors refer to Pandalus montagui as an Aesop shrimp, while others refer to it as an Aesop prawn.
Commonwealth countries, and Ireland, tend to follow British usage. Some exceptions occur in Australia, where some authors refer to small species of the Palaemonidae as prawns and call the Alpheidae pistol shrimp. Other Australian authors have given the name banded coral shrimp to the prawn-like Stenopus hispidus and listed "the Processidae and Atyidae as shrimps, the Hippolytidae, Alpheidae, Pandalidae and Campylonotoidea as prawns". New Zealand broadly follows British usage. A rule of thumb given by some New Zealand authors states: "In common usage, shrimp are small, some three inches or less in length, taken for food by netting, usually from shallow water. Prawn are larger, up to 12 inches long, taken by trapping and trawling." South Africa and the former British colonies in Asia also seem to generally follow British usage.
Shrimp is the more general term throughout North America, particularly in the United States, where it is the general term. In Canada, the terms are often used interchangeably as in New Zealand (larger species are prawn and smaller are often shrimp), but regional variations exist. In western provinces, prawn is almost exclusively the general term. The term prawn is less commonly used in the United States, being applied mainly to larger shrimp and those living in fresh water.