The sica was a short sword or large dagger of ancient Thracians, Dacians and Illyrians, used in Ancient Rome too, originating in the Halstatt culture. It was originally depicted as a curved sword (see the Zliten mosaic as well as numerous oil lamps) and many examples have been found in what are today Albania, Romania, Bosnia, Bulgaria and Serbia. It is also depicted on Trajan's Column; notably the Dacian king Decebalus is depicted committing suicide with one.
From a typological point of view, though there is a strong tendency towards standardization, the sica daggers can be organised in three main types that differ only by morphological aspects, not functional.
The first type is characterized by its solidity, sometimes with a broken figure immediately after the blade’s middle part, with a short and sharp point, with a slight curvature, a short handle cane, usually of a triangular shape, that has a hole for the fixing rivet, close to the blade. The blade has incised ornaments and a fuller deeply carved into it. These characteristics are not general, the only standing arguments for this type being the size and the shape approximately similar. 
The second type, not very different morphologically speaking, does not have the solid shape of the first type, but a longer blade, with a fuller, and keeps, most often, the haft's tongue short and of a triangular shape. 
The third type, more numerous, gathers a series of daggers characterized by a long blade, elegantly manufactured in most cases, by the ornamentation with incised circles and/or lines along the blade, the presence of the fuller, the haft’s tongue as long as the haft and a guard muff. These distinctive elements are found either all together, or some of the pieces present one or more of these characteristic features. The sizes show a relative standardization, somewhere around 30–40 cm length and approximately 3 cm width. From a chronological point of view, this type of dagger is dated mostly in the 2nd and 1st centuries BC. 
The distinctive shape was designed to get around the sides of an opponent's shield, and stab or slash them in the back. Since the thraex gladiator's usual opponent was the scutum (large shield) carrying murmillo gladiator, such a weapon as the sica was necessary to make the duel more even and exciting.
The daggers’ decoration, remarkably complex, offers them a sum of spiritual, artistically and symbolical valences. On the blade geo- metrical shapes were incised, but also eagles and snakes, in which case their schematization requires the existence of a certain "code", or they were an emblem that underlined the membership of a certain brotherhood at arms or a certain social status, as well possible to have a mystical/magical component included.
From the facts presented above, it can be stated that the sica dagger represents an important historical artifact which, due to its importance and the role it played in the Thracian world, contributes to the understanding of the social and military mechanisms of this society and, through the special spiritual dimension, to the reception of a new side from the religious mosaic of this people. From all the curved weapons used in the Thracian area, the sica daggers are the only ones that make the connection between the Southern Thracians and the Thracians North to the Danube, being spread the same on both sides of the river.
Origin of name
According to Dictionnaire des Antiquités Grecques et Romaines, tome 4, volume 2 (R–S), Paris, 1926, p. 1300, s.v. sica, the name Sica comes from Proto-Indo-European root sek-, meaning "to cut", "to section". Roman author Valerius Maximus, III, 2.12, said that sica was the Dacian name for the curved short sword/bigger knife of this type. In Albanian the knife is called today "Thika" which is similar to "Sica".
Although a short curved sword was used by several peoples around the Mediterranean the Romans regarded the sica as a distinct Illyrian weapon used by the stealthy 'assassin' (sicarius)
- The Illyrians (The Peoples of Europe) by John Wilkes, 1996, page 238, "Their principal offensive weapon was the single edged curved-sword, similar to the Greek machaira, a form of weapon that can be traced back to Bronze age times..."
- The Illyrians (The Peoples of Europe) by John Wilkes, 1996, pages 238–239, "Although a short curved sword was used by several peoples around the Mediterranean the Romans regarded the sica as a distinct Illyrian weapon used by the stealthy 'assassin' (sicarius)..."
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- www.enciclopedia-dacica.ro http://enciclopedia-dacica.ro/index.php?option=com_content&view=article&id=709&Itemid=377
- Dictionnaire des Antiquités Grecques et Romaines, tom 4, vol. 2 (R–S), Paris, 1926, p. 1300, s.v. sica