The word tunica is an almost direct translation from the Latin word tunica and is an ancient garment. It was worn for the first time by the Byzantine Greeks, but became more associated with citizens and non-citizens in the Roman Empire. Yet in other countries that had little contact with Rome or Greece until late, such as the Norse groups commonly called Vikings, people wore simple tunics. Perhaps the simple structure and design are responsible for its spontaneous evolution in different parts of the world.
The first style of tunic was really simple. It was a long garment, typically knee-length, and usually worn with a belt over the belt. It was typically characterized by short sleeves. Roman citizens could wear a toga over their tunic and Roman soldiers wore it under their armor. The garment had an unsewn part on the head, seams on the sleeve and seams on the side. If you think of a sack of potatoes with a slit to fit the head and openings on each side to fit the arms, you essentially have the garment.
The tunic gradually evolved into a long-sleeved look, which was perhaps very suitable for members of the army. Having to fight in the northern regions of Europe with a lot of snow would have meant that the soldiers needed extra heat. In addition, the Rome headquarters had its periods of cooler weather, which would make a garment with sleeves more comfortable.
Anyone in Rome could wear a tunic, but the colors and designs may vary indicating the welfare status. For the non-Roman citizen it would have been simple in the design, perhaps of raw wool or linen, and probably not dyed. A non-citizen could not wear a tunic with a toga as the togas were a sign of citizenship. There is some dispute about the colors worn by the soldiers. Some suggest that they were dyed red, or that only high-ranking soldiers could wear the red tunic. For the non-military citizen, they could be made in various colors or bleached to a faint white.
From the fifth to the eighth century, there are references to the Viking tunic. In fact, members of the Society for Creative Anachronism will tell you exactly how to make one if you plan to dress up as a Viking to attend their imaginative events. The Viking tunic may have been worn without a belt and is usually made of wool. Today, and in the past, it has become an important part of the priestly costume. Unlike the Roman or Viking tunic, the garment is long to the floor and is not belted.
You can also find many modern shirts built on this design, especially for women. A long tunic, which hits about mid-thigh, can be worn over leggings, stretch pants or jeans and have sides with slits. Trends towards adapting the garment a little better to the body have led to a flattering style, although many prefer to wear a wider one for simple comfort. A related garment is the caftan, which can be worn by men and women in some parts of the Middle East, and is mainly worn by women in the Western world. The caftan is considered a garment for relaxing in the West and can be long to the floor with a simple construction.