It is shoulder in general when talking about shirts, but I also found shoulder band when talking specifically about tank tops (as follows):
2xist Square Cut Tank Top - Style 3127



This 2xist tank top features a unique square-cut neck that shows off your upper body. This is the 50 Cent Tank Top.

Product Features:
* 100% extra soft cotton
* Extra wide shoulder bands for comfort and style
* Better fitting and more substantial than a basic tank top
* This 2xist tank top is ribbed and fitted for an athletic look
* 50 Cent Tank Top


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Note added at 45 mins (2005-11-08 08:40:45 GMT)
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Undershirts would be the general category, which includes short-sleeve, long-sleeve and sleeveless shirts. The term for "canottiera" to be selected depends on the target country. Tank top for U.S., singlet for Australia, vest for UK. See for example the following from Wikipedia:

In American English a tank top is a sleeveless shirt that can be worn by either sex. The term is also used in Australian English; however the term singlet is more common. The name derives from its resemblance to a tank suit, a style of one-piece women's swimsuit with shoulder straps. ("Tank suit" in turn refers to "swimming tank," an obsolete term for what are now called "swimming pools.")
Tank tops are often worn by athletes in sports such as Track and field and bicycling.
Tank tops have been popular warm-weather casual wear in the United States since the 1980s and are regarded as acceptable public casual dress in most locales there.
Elsewhere, the term singlet or vest may be used.
In British English a tank top denotes a sleeveless pullover.

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Note added at 59 mins (2005-11-08 08:54:49 GMT)
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Either way will work. I personally prefer undershirt with ... shoulder bands.