raglan sleeves
Raglan sleeves are sleeves attached to a garment via a diagonal seam from the armpit to the neckline (rather than a simple circular seam at the edge of the shoulder). They are often made in a different color than the torso of the garment and are constructed from one piece of material. Raglan sleeves are commonly used in sportswear, as they are designed to give greater freedom of movement for the arm and shoulder.
T-shirts with this type of sleeve are sometimes called "baseball shirts" due to their popularity of use as undershirts for baseball uniforms. Raglan sleeves became very popular in fashion outside of sports in the start of the 1970's.