The word tunic is a near direct translation from the Latin word tunica and is an ancient garment. It was first worn by the Byzantine Greeks, but became most associated with citizens and non-citizens in the Roman Empire. Yet in other countries that had little contact with Rome or Greece until later, like the Norse groups commonly called Vikings, people wore simple tunics. Perhaps the simple structure and design is responsible for its spontaneous evolution in several parts of the world.
The first tunic style was simple indeed. It was a long garment, typically about knee length, and usually worn belted with blousing above the belt. It generally featured short sleeves. Roman citizens might wear a toga over their tunic, and Roman soldiers wore them under their armor. The garment had an unsewn portion at the head, seams on the sleeve and seams on the side. If you think of a potato sack with a slit to fit for the head and openings on each side to fit the arms, you essentially have the garment.
The tunic gradually evolved to a long-sleeved look, which was perhaps very suited to members of the Army. Having to battle in Northern regions of Europe with lots of snow would have meant soldiers required extra warmth. Additionally, the seat of Rome had its periods of cooler weather, which would have made a garment with sleeves more comfortable.
Anyone in Rome might wear a tunic, but colors and designs could vary indicating social status. For the non-Roman citizen, it would have been simple in design, made of perhaps rough wool or linen, and was probably undyed. A non-citizen couldn’t wear a tunic with a toga since togas were a mark of citizenship. There’s some dispute about the colors worn by soldiers. Some suggest they were dyed red, or that only high ranking soldiers got to wear the red tunic. For the non-military citizen, they might be made in varying colors or be bleached to a soft white.
By the 5th to 8th 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’re planning on dressing as a Viking to attend their fantasy filled events. The Viking tunic may have been worn unbelted, and is usually made of wool. Today, and in the past, it became an important part of priestly costume. Unlike the Roman or Viking tunic, the garment is floor length and is not belted.
You can also find many modern shirts constructed on this design, most particularly for women. A long tunic, hitting at about mid-thigh, might be worn over leggings, stretch pants or jeans and have slit sides. Trends toward fitting the garment a little better to the body has resulted in a figure flattering style, though many prefer to wear a looser one for simple comfort. A related garment is the caftan, which may be worn by both men and women in parts of the Middle East, and is worn mostly by women in the Western world. The caftan is considered a lounging garment in the West and may be floor length with simple construction.