What is "dimachae"

Dimachae (from Ancient Greek διμάχαι from δί- di- (from δίς dis, "two, double, twice") and μάχη mache "fight") were Macedonian horse-soldiers, who also fought on foot when occasion required. Their armour was heavier than that of the ordinary hetairoi, and lighter than that of the regular heavy-armed pezhetairoi. A servant accompanied each soldier in order to take care of his horse when he alighted to fight on foot. This species of troops is said to have been first introduced by Alexander the Great.