A dunam or dönüm, donum was a unit of land area used in the Ottoman Empire and representing the amount of land that can be plowed in a day. The unit is still in use in many areas previously under Ottoman influence.
The legal definition was "forty standard paces in length and breadth", but varied considerably from place to place, from 900–2500 m². In many formerly Ottoman regions, it is now defined as exactly one decare (1000 m²).
The name dönüm, from the Ottoman Turkish دونمك / dönmek (to turn) appears to be a calque of the Byzantine stremma and had the same size. It was likely adopted by the Ottomans from the Byzantines in Mysia-Bithynia. In Arabic, the word is spelled دونم (dūnam) which is "a square measure (Iraq= about 2500 m²; Palestine= roughly, 1000 m²)."
Bosnia and Herzegovina, Serbia
In Bosnia and Herzegovina and also Serbia, the unit is called dulum (дулум) or dunum (дунум). It is equal to 1,000 square meters.
In Cyprus the donum is 14,400 square feet (1,338 m2). In the Republic of Cyprus older Greek Cypriots also still refer to the donum, although this is gradually being replaced by another local Greek Cypriot dialect word, σκάλες ['skales], rather than the mainland Greek word stremma. However, officially Cyprus uses the square metre.
Syria, Israel, Palestine, Jordan, Lebanon and Turkey
In Israel, Palestine, Syria (where it is called a dulum), Jordan, Lebanon and Turkey the dunam is 1,000 square metres (10,764 sq ft), which is 1 decare. Before the end of the Ottoman Empire and during the early years of Palestine, the size of a dunam was 919.3 square metres (9,895 sq ft), but in 1928 the metric dunam of 1,000 square metres (0.10 ha) was adopted, and this is still used.
The stremmax (Greek: στρέμμα, plural στρέμματα) is a Greek unit of land area, equal to 1,000 square metres, also called the 'royal' stremma. The name comes from a root meaning 'to turn', presumably referring to the amount of land that can be plowed/turned in a day.
The "old", "Turkish", or "Ottoman" stremma was approximately 1,270 m² (Λεξικό, 1998): it was the Greek name of the Ottomandönüm, which was in turn based on the Byzantine stremmax (see below). But Lapavitsas uses the value of 1,600 m² for the region of Naoussa in the early 20th century.
The medieval or Morean stremmax was different, somewhere between 900 and 1,900 m², depending on the period and perhaps even the type of land.
The Byzantine stremma was defined as the area of a square whose sides have a length of 100 Greek feet or 40 Greek paces. It is likely the ancestor of the Ottoman dunam/stremma.
^V.L. Ménage, Review of Speros Vryonis, Jr. The decline of medieval Hellenism in Asia Minor and the process of islamization from the eleventh through the fifteenth century, Berkeley, 1971; in Bulletin of the School of Oriental and African Studies (University of London) 36:3 (1973), pp. 659-661. at JSTOR (subscription required)
^Costas Lapavitsas, "Social and Economic Underpinning of Industrial Development: Evidence from Ottoman Macedonia", Ηλεκτρονικό Δελτίο Οικονομικής Ιστορίας
^Siriol Davis, "Pylos Regional Archaeological Project, Part VI: administration and settlement in Venetian Navarino", Hesperia, Winter, 2004 
^V.L. Ménage, Review of Speros Vryonis, Jr. The decline of medieval Hellenism in Asia Minor and the process of islamization from the eleventh through the fifteenth century, Berkeley, 1971; in Bulletin of the School of Oriental and African Studies (University of London) 36:3 (1973), pp. 659-661. at JSTOR (subscription required); see also Erich Schilbach, Byzantinische Metrologie (referenced but not seen)