Stremma

The stremma (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 Ottoman dönüm, which was in turn based on the Byzantine stremma (see below). But the exactly value varied by region.
The medieval or Morean stremma 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 dönüm/stremma.
Conversions
One stremma is equivalent to:
Metric

1,000 square metres
10 ares
1 decare
...