A tunic is a generic fashion term used to describe any number of different articles of clothing that start at the shoulders and reach anywhere from the wearers waist to ankles.
As a term used to describe an article of clothing, tunic can be dated back to the Byzantine Greeks where a form of the tunic was worn by most Greek citizens as part of their everyday wear. These tunics were garments that covered much of the body and thus were worn without pants.
Around medieval times some tunics became significantly shorter and closer in resemblance to a modern-day long-sleeved T-shirt than an ancient Greek toga while others maintain the length of older tunics but were worn with pants.
The modern-day tunic is no different from the tunic of the past in so much as there is still no common understanding of what a tunic specifically is or is not. It can be a long robe worn by clergy or a short shirt worn by club goers. The turn tunic is also used interchangeably with a non-buttoned shirt in some cultures.
Because of all this confusion surrounding what a tunic actually is it is better to not question someone when they describe something as a tunic as they are only as right or as wrong as their personal understanding of the term.