This is the women's AIRism short sleeve scoop neck t-shirt from Uniqlo, and one of my new obsessions/summer essentials.
AIRism is a special collection of clothing from Uniqlo that focuses on being "light as air" in that hot Japanese summer heat. The tag line is "comfort unlimited." This line has a variety of styles including various tanks, and tees, as well as leggings, slips, and lounge wear.
The material is a soft microfiber blend of 60% nylon, 30% cupro, and 10% spandex. It moves with you, dries quickly, breathes well, and is odour resistant. I wear it on its own, but this would work as an undershirt, too. As for the fit, I recommend trying this on before you buy it.
It fits much smaller than other tops from Uniqlo. I usually wear a small, because so many of their tops are very drapey and loose, but I wear a size large in this shirt. If you do get a smaller size, and have curves, the shirt will ride up and want to bunch around your waist.
This shirt is so comfortable and I get lots of complements on it. I like to wear it tucked into skirts. It's so versatile, and at only 900yen (or $12.90 in America) it's budget-friendly, too.