What "a T-shirt" is?
I would like to know if a T-shirt is used only to name the outer garment or also to name the undershirt.
In my dictionary, it appears as undershirt but only in American English. Then, I must suppose that "T-shirt" is an outer garment in British English and an undershirt in American English. Is that?
7 Answers
Well, it depends on how you use the T-shirt. If you live in a colder place, you can use a T-shirt as an underwear (I do it from time to time). I found the following definitions of a t-shirt:
a soft shirt with short sleeves and no collar (Longman Dictionary of Contemporary English)
a simple piece of clothing which covers the top part of the body and which has no collar and usually short sleeves (Cambridge Advanced Learner's Dictionary)
Let's not overlook the all important souvenier. I have T-shirts from every dive trip, concert and place of interest that I have ever been to. Here in New Hampshire, at this time of year, I wear a T over a thermal longsleeved shirt. To most here, a plain white cotton T is an undershirt which comes in regular or V neck. Otherwise it's the most popular casual shirt worn.
I have always considered a T-shirt to be an "outer" shirt, never worn under something else ( unless your wearing it under a jacket or sweatshirt ). In my mind it's just a shirt to "hang out" in with short sleeves and usually a logo/picture/design on the front, but not always.
T-shirt in American English!
Well, in Spain we say "camiseta". And we can be talking about "una camiseta" (outer garment) or "una camiseta" as an undershirt. We use the same word.
It is not usually necessary to specify if it is an outer garment or an undershirt unless the other person does not know it and then he/she asks: what type of T-shirt have you bought?. (¿Qué tipo de camiseta has comprado?). Then, I answer: una camiseta de vestir (outer garment). Or: una camiseta para llevar debajo de la ropa. (Una camiseta de debajo-very colloquial). Or: yo quiero comprar una camiseta de invierno (more formal. It is used to ask for a T-shirt in a shop). When you say "de invierno" the most natural thing is that you are referring to something to wrap up well. Therefore, you are talking about the underwear.
But it depends on the context. "Camiseta de invierno" can be also used for a outer garment. But, it is true that if you say to a shop assitant that you want "una camiseta de invierno", you are likely to want to say "underwear". But, if there are two types of "camiseta de invierno" (outer garment and underwear), then the shop assistant can ask you: ¿qué tipo de camiseta de invierno quiere? ¿para llevar encima o debajo de la ropa?.
It sounds a bit complicated, but with the passage of time .... Don't you think?
In British usage, a T-shirt is a T-shirt, whether you wear it alone, or under something else. If a garment is specifically designed for wearing underneath another shirt, we would call it a vest.
"Undershirt" is not a word that I have ever heard a native British speaker use 
If you check Wikipedia (http://en.wikipedia.org/wiki/T-s) you'll find that today's T-shirt originated with the knit undershirt. Speaking from a US perspective, I call a plain, solid colored knit shirt, generally worn underneath a woven shirt, an "undershirt" and a decorated knit shirt that can be worn alone or underneath another shirt a "T-shirt." The only difference in my mind is that the T-shirt has decoration (such as a printed or embroidered picture or logo). They are really the same basic garment. I am wearing a T-shirt underneath a sweatshirt right now!
