Flannel is usually woven from fine cotton yarn and worn on the skin. It is a underwear.
Undershirt ; It can be in short-sleeved, long-sleeved and suspended varieties as well as in varieties such as cycling neck, V-neck, wide collar.
Which is Right
Some people defend that undershirt is the correct expression, while others say that the expression athlete is correct. advocates. Actually, both can be said to be correct. We can say that the athlete is a type of undershirt. The fact that we think of the picture of underwear when we are told rather than determining which one is correct is an indication that there is no need for such a distinction.