Stadsholmen is the historical name of an island in the centre of Stockholm

Stockholm

Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

, Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

. Together with the small islands Riddarholmen

Riddarholmen

Riddarholmen is a small islet in central Stockholm, Sweden. The island forms part of Gamla Stan, the old town, and houses a number of private palaces dating from the 17th century...

 and Helgeandsholmen

Helgeandsholmen

Helgeandsholmen is a small island in central Stockholm, Sweden. It is located north of Stadsholmen, and east of Strömsborg, with which, together with Riddarholmen, it forms Gamla stan, the old town of Stockholm...

 it forms the Old town of Stockholm

Gamla stan

Gamla stan , until 1980 officially Staden mellan broarna , is the old town of Stockholm, Sweden. Gamla stan consists primarily of the island Stadsholmen. The surrounding islets Riddarholmen, Helgeandsholmen, and Strömsborg are officially part of, but not colloquially included in, Gamla stan...

.
Its name literally means Island of the City and it is also the location of the Royal Palace.

The name Gamla stan

Gamla stan

Gamla stan , until 1980 officially Staden mellan broarna , is the old town of Stockholm, Sweden. Gamla stan consists primarily of the island Stadsholmen. The surrounding islets Riddarholmen, Helgeandsholmen, and Strömsborg are officially part of, but not colloquially included in, Gamla stan...

(Old Town) can also refer to the island itself, as the name Stadsholmen is rarely used in daily speech.