The damson (/ˈdæmzən/), damson plum, or damascene (Prunus domestica subsp. insititia, sometimes Prunus insititia), is an edible drupaceous fruit, a subspecies of the plum tree. Varieties of insititia are found across Europe, but the name damson is derived from and most commonly applied to forms that are native to Great Britain. Damsons are small, ovoid, plum-like fruit with a distinctive, somewhat astringent taste, and are widely used for culinary purposes, particularly in fruit preserves and jams.

In South and Southeast Asia, the term damson plum sometimes refers to jamblang, the fruit from a tree in the family Myrtaceae. The name "mountain damson" or "bitter damson" was also formerly applied in Jamaica to the tree Simarouba amara. Terminalia microcarpa and Chrysophyllum oliviforme are also sometimes referred to as damson plums.

The name damson comes from Middle English damascene, damesene, damasin, damsin, and ultimately from the Latin (prunum) damascenum, "plum of Damascus". One commonly stated theory is that damsons were first cultivated in antiquity in the area around the ancient city of Damascus, capital of modern-day Syria, and were introduced into England by the Romans. The historical link between the Roman-era damascenum and the north and west European damson is rather tenuous despite the adoption of the older name. The damascenum described by Roman and Greek authors of late antiquity has more of the character of a sweet dessert plum, not fitting well to the damson plum. Remnants of damsons are sometimes found during archaeological digs of ancient Roman camps across England, and they have clearly been cultivated, and consumed, for centuries. Damson stones have been found in an excavation in Hungate, York, and dated to the late period of Anglo-Saxon England.

The exact origin of Prunus domestica subsp. insititia is not known with certainty. It is often thought to have arisen in wild crosses, possibly in Asia Minor, between the sloe, Prunus spinosa, and the cherry plum, Prunus cerasifera. Despite this, tests on cherry plums and damsons have indicated that it is possible that the damson developed directly from forms of sloe, perhaps via the round-fruited varieties known as bullaces, and that the cherry plum did not play a role in its parentage. Insititia plums of various sorts, such as the German Kriechenpflaume or French quetsche, occur across Europe and the word "damson" is sometimes used to refer to them in English, but many of the English varieties from which the name "damson" was originally taken have both a different typical flavour and pear-shaped (pyriform) appearance compared with continental forms. Robert Hogg commented that "the Damson seems to be a fruit peculiar to England. We do not meet with it abroad, nor is any mention of it made in any of the pomological works or nurseryman's catalogues on the Continent". As time progressed, a distinction developed between the varieties known as "damascenes" and the (usually smaller) types called "damsons", to the degree that by 1891 they were the subject of a lawsuit when a Nottinghamshire grocer complained about being supplied one when he had ordered the other.

In addition to providing fruit, the damson makes a tough hedge or windbreak, and it became the favoured hedging tree in certain parts of the country such as Shropshire and Kent. Elsewhere damsons were used in orchards to protect less hardy trees, though orchards entirely composed of damson trees were a feature of some areas, notably the Lyth Valley of Westmorland and the Teme Valley in the Malverns, and indeed damsons were the only plum planted commercially north of Norfolk.

There is a body of anecdotal evidence that damsons were used in the British dye and cloth manufacturing industries in the 18th and 19th centuries, with examples occurring in every major damson-growing area (Buckinghamshire, Cheshire, Westmorland, Shropshire and Worcestershire). Stories that damsons were used to dye khaki army uniforms are particularly common. However, a 2005 report for conservancy body English Nature could find no documentary evidence within the dyeing industry that damsons were ever a source of dye, noting that use of natural dyes declined rapidly after the 1850s, and concluded that "there seems no evidence that damsons were used extensively or techniques [for using them] developed". The main recorded use of damsons in the industrial era was in commercial jam-making, and orchards were widespread until the Second World War, after which changing tastes, the effect of wartime sugar rationing, and the relatively high cost of British-grown fruit caused a steep decline.

The damson was introduced into the American colonies by English settlers before the American Revolution. It was regarded as thriving better in the continental United States than other European plum varieties; many of the earliest references to European plums in American gardens concern the damson. A favourite of early colonists, the tree has escaped from gardens and can be found growing wild in states such as Idaho.

The main characteristic of the damson is its distinctive rich flavour; unlike other plums it is both high in sugars and highly astringent. The fruit of the damson can also be identified by its shape, which is usually ovoid and slightly pointed at one end, or pyriform; its smooth-textured yellow-green flesh; and its skin, which ranges from dark blue to indigo to near-black depending on the variety (other types of Prunus domestica can have purple, yellow or red skin). Most damsons are of the "clingstone" type, where the flesh adheres to the stone. The damson is broadly similar to the semi-wild bullace, also classified as ssp. insititia, which is a smaller but invariably round plum with purple or yellowish-green skin. Damsons generally have a deeply furrowed stone, unlike bullaces, and unlike prunes cannot be successfully dried. Most individual damson varieties can be conclusively identified by examining the fruit's stone, which varies in shape, size and texture.