The acinaces is of Scythian origin, but was made famous by the Persians, and rapidly spread throughout the ancient world. Its influence can even be seen in the design of Chinese weapons such as the jinglu sword. The Romans believed this weapon originated with the Medes.
The acinaces is typically 35-45 cm. (14-18 in.) in length and double-edged , and although there is no universal design, the guard may be lobed with the hilt resembling that of a bollock dagger, or the pommel may be split or of the "antenna" type. Interestingly, the scabbard as much as anything else defines the acinaces and usually has a large decorative mount near the opening allowing it to be suspended from a belt on the wearer's right side.
Since the acinaces seems to have been a thrusting weapon, and since it was typically worn on the right, it was likely intended to be suddenly drawn with the blade facing down for surprise stabbing attacks.
Ancient texts say very little about the acinaces, other than that it was a type of "Persian sword." Because of this, authors writing in Latin throughout history tended to equate the word with whatever type of weapon the contemporary Persians were using. Thus, it is frequently used in medieval Latin texts to mean scimitar or the like, a meaning it still retains in scientific Latin. Paulus Hector Mair even goes so far as to translate dussack as acinaces, because it is curved like a scimitar, and likewise in the works of Jesuit authors describing Japan, acinaces is used for katana.
However, the Persian shamshir is a relatively recent weapon, and did not exist in antiquity. The Achaemenid-era Persians made use of more than one kind of sword. Ancient Persian art typically shows the king's bodyguards and important nobles wearing ornate diagonal daggers. Greek art, on the other hand, frequently shows Persian soldiers using the kopis. One must therefore do some detective work to figure out which type is the acinaces.
A ritual use of acinaces, offered as a gift to the sea by the Persian king Xerxes, is also mentioned by Herodotus (History, VII, 54), in the ritual contrition scene following the episode known as Flagellation of Hellespont.